When I started the car and tried to drive, it would rev, but there was no power behind it. Technically, it was driving, but there was only very weak acceleration -- couldn't get up to speed, could only get to maybe 25 mph. It wasn't cold weather or anything, either. I drove around the block, stopped back at home, turned off the engine, waited awhile, and when I turned it back on, it seemed to be working okay. Have had the problem one time since, but otherwise it's been fine.