I don't know if these cars have one or not, but research replacing your MAP sensor. My 89 Tempo had one that went bad, and it would constatly rev, etc, on its own, and die out if you didn't keep your foot on the gas. Chances are if a good sized chunk of gravel got into your pan, it woudln't get pulled into the engine. I wouldn't want to find out though. BTW, why did you take the whole drain pan off when you changed the oil? And maybe it is just chunks of old gasket if you didn't replace it w/ a new one properly.
