[SuperHart]

Perhaps some of this is reiteration but here goes: there are usually two possible problems to check out with the situation you describe assuming that your distributor has a vacuum advance unit. First, as previously mentioned, a sticky mechanical advance system could be the culprit. Rather than disassemble the distributor to check it out, check the vacuum advance module first. Unplug the hose to the vacuum module from the carb and pulg the hose. Check and adjust the idle speed to around 800 RPM. The plug the vacuum advance back in. The idle speed should increase maybe 200-500 RPMs and stay there. Try recreating the scenario you describe byt giving the motor some throttle and then returning to idle. A faulty or sticking vacuum module might initially hold vacuum keeping the idle high but lose it due to a faulty diaphragm and cause the idle speed to drop. Replacing the vacuum module will correct such a probolem. The purpose of the vacuum module is to provide advance at idle which supposedly reduces oxides of nitrogen emissions and helps the motor to idle cooler

If the vacuum module checks out OK, the it's time for the timing light to check the mechanical advance. Connect your timing light and disconnect the vacuum module and plug the hose. Try to recreate the scenario you describe about the dropping idle. You should already know what your initial timing is (10-12* approximately). If you seem to have more timing than that when you first get to idle. and the timing decreases back to initial setting when the idle drops low then you probably have an advance weight that is sticking or a faulty mechanical advance spring. Either problem is easy to correct but will require partial disassembly of the distributor, maybe even while the unit remains in the car.