loping and leads...issues...

From the sounds of it he is either one lead dominant or your body and hand positions are confusing to him.  He's what I would try.
 
If you have acess to an arena or open place to canter your horse safely there are several different excersises to work on getting the proper lead.  First I would really work on body and hand positions so that even if you pick up on the bit, he yeilds to the bit but doesn't break gait from the trot to walk until you relaxe you body and ask.  Chances are he is getting confussed by the stoping and starting, when you bend him into a circle that will slow his canter down more than picking up on the bit.  REally focus on how you are sitting in the canter because your hips have to acutally make a circle to accomidate the suspension of the stride.
 
When you have got him warmed up and ready get him in a nice soft trot down the rail.  Don't drive him with your hands or upper body just you hips when your ready.  As you are coming around the first corner (lets say we are wanting the left lead) place your right leg back to yeild the hindquarters, even if your horse doesn't know much about this its still ok because it puts your body in the right position to ask for the left lead departure.  With your leg still there, lift your left rein up to lift your horses inside shoulder.  Picture it as being conected to a string, you lift you hand and the shoulder will come up.  Now you can change from the posting or sitting trot and start using your hips and kiss to him as you are coming around the corner to the left.  Most horses will take the correct lead if they know what direction you want them to go in.
 
When he takes the lead let him move out for a while bringing him into a wide circle and then down to the trot.  Wait till his is calmed back down and relaxed and try for the other lead.  If he is more dominant on one side or the other when he takes the correct lead its important to let them keep cantering to build the muscles on that side of their body.
 
Another thing to try if he is still droping to a trot when you try and bring him to circle is bit him up and put him on a lounge line attatched to the bit.  Be consistant and use the direction to help him take the lead and make him canter both directions for a while.  If he is pulling on your hand with the bit just keep jiggling it and cuing for him to canter till he canters and stops pulling.  This will get him to yeild to the bit pressure without dropping gate so much.  If he is really lead dominante one way make sure that if you get him on the other lead to let him lope for quite a while to show him that he did the right thing and to build those muscles up.  Then get on and try it undersaddle remember its Leg, Rein, seat, Kiss.  For the horse that hasn't done this much take it slow and break it down by placing your leg back waiting till you feel them thinking, lift the rein up, start circling your hips and kiss as you come around the corner.
 
HOpe this helps give you some ideas.

Okay i really agree with what trainer 101 says, but i think going back to a couple of basics will help you A LOT! So one thing i do with my student who has the same problem (her horse is an ex endurance horse) is i like to just jog her horse in circles, i mean tight ten foot circles. While doing these circles, ask your horse to soften up and bend around that circle. His nose should be traveling around that circle BEFORE his shoulder, or hip. Then change directions and do the same thing the other way. I would do this until your horse gets responsive and lightens up. Once he is light and soft in the mouth, i would ask for the lope. Set him up just like trainer 101 says where you show him the direction you want to go (like in another circle but bigger, maybe a 40 foot circle) and yeild his hind quarters. Once he picks it up and has the correct lead, hold him in that arced position. It is very hard for them to get out of the lead you ask for if you are going in a circle. If your horse picks up the wrong lead, bring him back down to your circles at the trot and then ask again. If he is going fast at the lope, chances are he wants to get into the lope as soon as he can, so make him wait and slow him down. Once he relaxes again, ask for the lope. And remember, once he goes in a few circles without switching, really reward him.  I know there is alot more to this but i hope this helps. Just get that foundation....remember, if you cant do it at a walk or trot, you wont be able to do it at the lope so remember smalls steps and you'll get alot further.

alright so i found out the problem isn't diablo its me []
my body language is telling him to go faster cause i lean forward, and tense up my shoulders i had a cousin take a quick video of me and diablo loping and thats the issue, so just cause i could i tried sitting back and a&nbsperfect even lope came out of diablo, *wipes brow* thats a relief, i just have to work on my riding a little, i also found out that i am left side dominate, (even though i write with my right hand.........) i can't seem to do as much with my right leg as i can with my left so i've got to work on building my muscles and his i dont usually work in the right lead cause it feels weird to me...
 
i also did the trotting down the side and asked for the canter at the turn and he immediatly went into the right lead so i thank you trainer for that
buckshot thanks! diablo and i do alot alot alot of circles and the bending has paid off, though he banks like an airplane around turns its crazy
thanks for your help both of you!
 
 

 Im no expert but thought this might help too. If your heading straight it helps to very slightly tip your horses nose to the opposite side from the lead you want just before you ask for the canter.