[denisond3]

The most common reason for this would be a sticking "idle air control valve". When they get old, replacement is called for. They cant really be cleaned, as they as still old.
I had a similar problem on my 91LX, even after replacing the i.a.c.v. I put a think piece of steel to increase the idle speed when the i.a.c.v. was at its minimum. It was a tiny chunk of a dual edge razor blade, glued onto the throttle valve 'stop' with permatex gasket maker. Its possible that the throttle valve pivot was worn. The car had over 250k miles on it.