I haven´t personally dealt with strokes(as far as I know) but have dealt with seizures from mites which pretty much look like involuntary spastic motions. There is more info on seizures at http://pub10.ezboard.com/fguinealynxfrm6.showMessage?topicID=52.topic
This is from Vicki of Jack Pine Guinea Pig Rescue. She has had experience with seizuring pigs:
The killer seizures did look different from mite seizures. With mites, it generally starts out with the pig swinging his head around towards his back - like he´s trying to chew at himself. Then he starts to stiffen up, still in that curled shape. The head may go back, and he will sometimes curl around to the other side. The eyes roll back, sometimes they will close and the legs will paddle. Mite seizures usually don´t last very long, though it may seem like eternity to a concerned owner that doesn´t know what´s going on. Cavies usually recover quickly from a mite seizure and act like nothing happened.
The seizures that killed my pigs looked more like grand mal seizure dogs sometimes get. The whole body stiffened up, head back, legs extended. I believe the body was kind of quivering. I think the eyes were open through the seizure (I remember tapping the corner to see if the pig was aware of me), though probably rolled back. There wasn´t the curling like you get with a mite seizure. With one pig it went on just about nonstop for hours, with a couple short breaks where she´d go limp - but then start seizuring again. I was unable to get medical help for her. Tried some torbugesic, didn´t do a thing. The 2nd case, a boar, started out with a bad bloat and then ended up seizuring. Had him to the vet, they tried to control the seizure with diazepam (Valium) injections. The injections had very fleeting affect.
Because the pig you describe is shaking it off once the seizure is over, and I assume not seizuring for very long, I would definitely look into it being caused by mites. Even with treatment the seizures can keep occuring for quite some time. My first rescued cavy, Harry, had severe mite infestation. At first she was seizuring often, seemed like every hour or so. As the ivermectin treatments started taking affect the seizures gradually happened less often, but it was weeks before they stopped completely. There is an article about Harry, My First Rescue, at my website (www.JPGPR.com).
If it´s not mites, it can be extremely difficult (if not impossible) to figure out what is causing the pig to seizure. A thorough veterinary exam is needed, preferably by a vet experienced with guinea pigs. The vet might want to try some simple things like changing diet or bedding to see if the cavy is possibly reacting to something in it´s environment. I have heard of dwarf hamsturds "having fits" when kept in cedar bedding. Is there any poisons the cavy can be getting to? Cleaning solutions used on the cage or accessories that aren´t fully rinsed off? Anything topical used on the coat or skin? Plants he might be tasting? Any possibility of mold in his pellets or hay? Toxic weeds in his hay?
Vicki - Jack Pine Guinea Pig Rescue, MN