1989 Excel Stalling

I have a 4 speed manual transmission. The car recently started stalling after warm up. It idles good but sometimes the idle drops for no apparent reason before it stalls. I have checked for any vacuum leaks and found none. I also noticed that the idle speed doesn't go up when I turn on the headlights or hit the brake pedal. The idle up relay doesn't seem to be kicking in. Fuses are all good. Battery is brand new. I raised the idle screw enough just to keep it running but it still stalls. I noticed that if I goose the pedal, the anti stall dashpot does not get vacuum. I was wondering if the computer thinks its still in open mode because of an O2 sensor or maybe the TPS is faulty. Help!