Cat cramping? seizing?

Today I looked over and noticed my cat was leaning up against a wall. At first I thought he was doing his normal rub-on-everything routine, but the more I watched, the more distressed he sort of seemed. I noticed he was struggling to stay upright. He came free of the wall toward me and began to stumble, legs crossing as he tried to gain footing until he toppled over. He remained on the floor in a position that, to the casual observer, would have implied a sort of.. cramp throughout the body entirely. His back arched a bit, head slightly reared back and tail curling up as well. Legs seemed to seize, toes curled somewhat. I felt his neck and legs as I pet him, talking to him and trying to get him to relax. The muscles felt cramped, but I won't pretend to be an expert. I could be wrong.
   
No mews or sounds of pain, though his eyes were wide and a bit dilated with the distress. He didn't seem to be panicking, and after I went to his side and was trying to calm him, he did start to purr. He's always been strange about purring, and the only time he -doesn't- do it is when he's terrified. He purrs when he's angry, constantly when in human contact, and often just when sitting around by himself.
   
The muscle "spasm" or "seize" (I really don't know what to call it" passed after about 30-50 second and afterward he was acting completely normal. Walking without a care, jumping on the bed, crawling on my lap to beg for petting. I hoped it was a passing occurrence.
   
10 minutes later he did it again. It's been 20 minutes of fruitless internet research and he has not done it again. Nothing seems to be directly causing it. Nothing unusual happened today, nothing strange was ingested, no injury caused, no stress.
   
I will be taking him to the vet, but I don't have access to my car today or tomorrow due to mechanical issues and I'm in an awkwardly stranded sort of situation at the moment, so this is my desperate attempt to seek some information in the meantime.