I'm no expert, and I agree it sounds a lot like a behavior problem. Which ends up with lunging, and lots of ground work.
I just wanted to add: Have you looked into tying-up? If the horse is not currently doing it, the vet can't diagnosis it. The symptoms can vary trememdously.
The reason I ask is because Christy was super girthy, she even got to the point where she wanted to toss her saddle off before I could cinch it. Then, after 15 - 20 minutes of riding she suddenly started bucking, wanting to drop and roll, rearing, kicking out... All her tack fit well, I spend months trying to figure out what was going on. Finally the vet said she might be tying-up. Apparently instead of getting muscle tremors or weakness, she colics. Very painful colic.
I have put her on a low sugar, low carb. diet high in fat. She has been on this diet for 2 months and I have no more problems. The vet wanted me to bring her to him ASAP at the first sign of colic, well, no colic. When they are tying-up a blood test can determine which type etc... Otherwise, the vet just has to guess. Under the circumstances, I'm fine not knowing which type.
Anyway, I wanted to mention this... Just in case.