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Scott-
How do you know that it is running rich?
do you have an air/fuel meter in the car? if so, is it hooked to the same O2 sensor as the DME, or a seperate sensor?
I have a theory that revolves around either a faulty O2 sensor, or a faulty coolant temp sensor. are you sure the engine goes rich? maybe it is really going lean, usually, going slightly lean causes the rpm to jump up.
A little background on ECU operation here. all ECU's are different, i'm not 100% sure how the Bosch Motronic DME works, so all this info is generic, i hope it helps in some way. During warmup, the engine is in open loop mode. It ignores the O2 sensor. it determines injection time, and spark advance off of programmed databases (also refered to as maps) based on the inputs from the rest of the sensors on the engine, these are: TP (throttle position) sensor, the AF (air flow) sensor, the ACT (air charge temp) sensor, and the ECT (engine coolant temp) sensor. Once the engine warms up (determined by some combination of elapsed time, ECT reading, and valid readings coming from the O2 sensor) Ususally, once valid readings start coming from the O2 sensor, it jumps into closed loop mode. Once the ECU believes the engine has warmed up, it starts to consider the O2 sensor reading, and adjusts points on the maps richer and leaner based on the O2 readings, up to a maximum correction percentage. It goes so far as to believe the O2 sensor before it believes other sensors, and corrects to make the readings from the O2 sensor good, even if the info from the O2 conflicts with other sensors.
20 minutes is pretty long before it kicks into closed loop mode. If your O2 sensor is fouled, it could be reading either too rich, or too lean, causing the ECU to correct for a problem that doesn't really exist, which throws off the mixture. OR, the computer goes into closed loop mode, but the ECT sensor says the engine is still cold. The correction that the O2 sensor needs conflicts with the ECT sensor that says the engine still needs a overly-rich warmup mixture, and the ECU sets the mixture somewhere inbetween normal, and overly rich warmup mixture (which would be somewhat rich)
Not quite sure if that will answer your question, but some of that theory might help you narrow down the problem.
Mike
p.s. as AFJuvat said, silicone will contaminate an O2 sensor. even an extremely small amount will destroy a sensor very quickly. DO NOT use silicone ANYWHERE where vapors can get sucked into the engine, this means intake gaskets, throttle body, AF sensor, intake boots, or air box, or air filter. If there is silicone gasket material in any of these locations, this is probably your problem.
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