If this started happening over time, you more than likely have a gunked up throttle body. A 1.8L DSM will run decent with a leak in the intake or a faulty MAF. The problem I am thinking is reduced airflow or a bad Idle Speed Control motor. Try removing the intake pipe from the throttle body and use a rag and carburetor / throttle body cleaner and clean all the oil and grime deposits away. Make sure to hold the throttle plate open by hand and do a thorough job at the top and bottom where it seats when closed. This is good preventative maintenance that should be performed about every 50,000 miles, more frequently if it's burning oil. Next, once you've ensured the throttle body is clean, start the car and turn on the A/C. It does not matter if you even have the A/C hooked up / charged. With a fully functioning A/C, the idle should raise slightly and then drop back down. If the A/C is not connected, it should raise to about 1,000 rpm's and stay steady. Just out of curiosity, have you recently replaced the alternator. I did on mine and it had a bad habit of dropping the idle too low under these conditions (engine warmed up, headlights on, brake lights engaged and clutch pushed in) The solution was to drive the car and let the computer's memory relearn after disconnecting the battery. This was odd though because it seemed as though it took it 2 months to relearn and this is a car that is driven 15 miles back and forth to work 5 days a week. Let us know what you try and the results and we'll advise from there.
