Just a little FYI..... I had a Chevrolet car with a coil in the distributor, and I began having issues JUST LIKE YOU ARE DESCRIBING!!! I used to have to pull over and let the coil cool down, and I changed it and wha-laah! FIXED! You could very well have a coil issue. Check your plug firing and see if its strong when it acts up. Check it when its NOT acting up, and you should be able to tell then. I hope you get it fixed.