I have a 1995 DeVille my mother gave me with only 17k. About 50% of the time, the engine idle fluctuates between normal and about 1800 rpm. Ocasionally it cycles back and forth. Does anyone have an idea what could cause this?
Any help will be appreciated.

Very possibly a dirty throttle body.

Already had fuel injector(s) and TB cleaned...thanks for the suggestion though.

Bad ISC motor?

Thanks for the tip...new isc motor cured problem.