Almost sound like he might be 'crossfiring' , which is loping on different leads fore and hind or just plain travelling on the wrong lead.
Very rough to ride...had a horse that used to do that on occasion...not nice.
Best bet with that is to slow things down, and start working on getting him travelling in the proper lead again.
Find a nice safe area for you to work in, and start doing some figure 8's at a walk. As he settles in to the work, ask him to step up to a trot in that pattern.
Let him keep the trotting gait until he starts to relax while he's doing it (with high mettled horses this can take some time, but generally I find it's better to let them work their edge off before moving up to the lope).
Once he's confident and travelling easily with the trotting, gently ask him to lope when you're right where the two circles of the figure eight meet.
Keep your inside rein up (and therefore the outside rein lower), as this discourages the horse from dropping his shoulder and picking up the wrong lead.
If he's in the proper lead, his inside shoulder (to the circle in the direction you're travelling) should be leading first.
If he starts his lope on the incorrect lead, just slow him back down to the trot, and then cue him again as you approach the circle crossing.
This can take a while with some horses, as some come by it naturally, but others just seem to have two left feet when it comes to leads.
Well, that's about all I can think of, off the cuff, but then I'm sure no professional trainer. There's lots of folks on here with more know-how that will hopefully add more ((hint - hint to the horsemen and women out there)).
Here's a link that helps explain leads (it has a pop-up ad, but the content is pretty worthwhile irregardless):
http://horsetales.bravehost.com/training.html
Take care and good luck getting him smoothed out.