Now we have a Mercury problem - help!

Ok, Merc went up for sale, and after lots of emails, a few people wanted to come ride him. Because of my schedule, it took a few weekends until I was in town.
I wasn't riding him much, and a couple days before the first rider, I lunged him, the next day rode him, and he was his naughtiest ever.
The first rider came out, and although she had a nice calm demeanor, he was really unpleasantly naughty with her too. (I'll define naughty in a minute.) When they left, I lunged him, and he worked himself into a lather, and I had noticed he was chubby when someone else was riding him (his first chubby ever in 4 years!), and so I cut back his grain and hay, figuring he was just hot.
Unfortunately, I couldn't ride for three days, so three days later (this Monday), I lunged him (he lunged himself into a foamy lather), then rode him, and he was great until I asked for lateral work, then he was naughty, but we worked through it and ended fine.
Tuesday he was mellow on the lunge line, mellow under saddle, not very resistant for the lateral work, and we even popped a couple fences.
He got Wednesday off because of my pesky job.
Tonight he was the worst horse I've ever ridden.
He made me so scared I was sweating.
He deliberately tried to buck me off twice, which wasn't the worst.
He was erratic and difficult - throwing his head in the air, balking, jigging, running sideways, trying to back into the wall, trying to slam my leg into the wall, doing some half-rear things that weren't quite rears but were definitely front end oriented.
Usually, when I ask him to go on the bit, I squeeze the reins like a sponge. He would just throw his head up in the air and open his mouth and nothing I could do would get his head back down. He'd also run backwards.
He's actually quite athletic.
A smack on the whip would usually result in running sideways or backwards. The light leg tapping working up to leg slamming was just ignored.
And this is - I would ask him to walk and then stop and then walk again. EASY stuff.
We were fighting almost two hours. He had been mellow on the lunge, did one quick head toss when I first started riding, and then was good all the way through trot (with lots of serpentines and bends), and right lead canter. When I changed direction to do left lead, all this nonsense started, and he just went downhill.
I finally - after a huge struggle - got enough of a walk that I felt I could get off (without rewarding him for being bad), put him on the lunge line, and he was already - from fighting me, pouring sweat. And he lunged himself into a lather, I got back on, and then we had intermittent success. He'd walk a couple laps, on the bit, no problem, and then I'd ask for a halt and he'd just start running around sideways. Or we'd walk, no problem, I'd ask for a halt, get it, and then when I asked for the walk again he'd run around sideways.
It was absolutely terrible.
I have no idea why he's doing this if he's not hot, why he hasn't done anything like this before in the 4 years I've had him, or what to do about it. I'm not comfortable riding him alone if this is how he's going to act, and I certainly can't sell him like this.
I'm going to have the vet out next week to check for soreness and check his teeth, but he's not the least bit sore, gimpy, or sensitive, and he's not the kind of horse to stoically hide it.
He has a completely different personality. The only other thing I could think of was that I'm asking him for more because I'm learning more with Willig. But over the past 4 years, we've been on a slow and steady up, and he's never fought me - even when I ask something he doesn't know - he doesn't fight it.
Any thoughts? Suggestions? I am at a complete loss.