I have a well pattered but green (very limited time off the place) 5 yo I havent felt has been ready to start exhibitioning consistantly (partially due to my own lack of consistency with her thanks to my job) for a couple years now.  She's extremely athletic/catty, but has always been VERY fractious and felt "rough" to ride with a tendency to get light in her front end or try to buffalo you when she didnt want to do something.  I always chalked it up to lack of wet saddle blankets and exposure, but I recently switched feeds from Nutrena SafeChoice to Bluebonnet Total Advantage and noticed a huge improvement in her fluidity and attitidue.  Then last night she was a royal rip and I got off to lunge her and noticed her back feet landing at practically the same time when loping either direction.  As noted in a previous post, I've recently had trouble with her switching or losing her correct lead leaving the second barrel.  She's fine on #1 and #3, but it doesnt matter if we're going right or left, she loses that lead coming out, pushing off to #3.  WIth her not doing it on #3, I assumed it wasnt a soundness issue, but seeing her back feed land as they did last night I can't help but wonder... That being said - where do I start?! Are some horses just short strided in their back end (as she has always felt)?  Does it sound like a stifle issue?  Back?  Hocks?  I'm clueless here but I'm about at my wits end with her and despite how dang athletic and talented she is, if I don't figure something out soon it's time to do something different.  Any thoughts/suggestions would be apprecaited! :)  

I went through this exact issue with my 5 year old, oddly enough! He was on Seminole Dynasport (textured feed), and I switched him to Tribute Kalm N EZ - no longer a fire breathing dragon once we got away from the sugars. However, the only behavioral vice that remained was that he'd randomly stop and throw a buck when loping on his left lead, for no apparent reason. He also did this hopping lope you are mentioning, almost as though both back feet could have landed in the same 5 gallon bucket. 

The vet did an ultrasound and found slight inflammation in his stifle. He said it was probably due to being up for much of the winter since it was so rainy (I have a small pasture currently, and I don't turn out in a mud hole), combined with a little less work than would be ideal, and possibly an odd growth spurt. We injected that stifle, he's been great ever since, and the vet said it will likely resolve completely. I'm moving him out on 5 acres to be sure we don't go through this again next year! 

Fingers crossed yours can be fixed as quickly as ours was!