I second Cousin It's diagnosis... i had the same thing recently and it was a leaking throttle body gasket.
with the mods you mentioned, if you did them yourself recently before this happened, there may be a vaccuum leak somewhere.
take the intake tube off and put your hand over the entire opening with your car running.... if it still runs its a definite vaccuum leak.
dont play with any of the factory settings until you know where the problem is!!!