Thanks for the help, guys. I will check for both the misfiring cylinder and the water pump. Dave C Klasse, I haven't noticed any RPM fluctuations, but I'll keep an eye out the next time the thumping rears its ugly head (hasn't been doing it lately). One thing I can't stand about the dealer is their refusal to believe that there is something wrong with your car unless you can reproduce the symptoms on the spot (which, of course, never fails to be impossible). Assuming it is one of these two things, what would you estimate repair costs to be? Should I be worried about any permanent damage to the engine? Or is it something that I can have tended to at my convenience?