I had the same problem on my old 91 Chevy S10 Atx. It either idles real low to the point of stalling or it goes real high to you think its going to put a piston through the hood. Check you transmission fluid!!! Apparently I had a ever so slow radiator leak where the tranny fluid goes in and i was wicked low on tranny fluid. The engine surges cause it senses that its low on fluid and to keep it lubricated it surges to speed up the amount of fluid thru the system back to the tranny itself. If your tranny fluid is fine, then the pvc hose that went to your air box might have been the problem. Irregular air flow will cause the engine to do some weird things. I'm assuming that you replaced the oil that you lost right?
If none of that works I would say do a complete tune up on the engine. Check your plug wires, plugs, air filter, oil filter, new oil, also check your fuel pump.
If its not any of that I have no idea man.
~Dan
