Throttle position sensor for '90 V8

[QuickAudi at aol.com]

Symptoms: when it rains, the engine idles horribly and has even died.  When I start the car, rain or shine, it idles roughly, even in Drive, for about 5-10 seconds, then it is fine.  It does not get better when it rains.  It has been doing the rough idle thing on start up for some time; the rain one is a new comer.  As far as I can tell, it doesn't hurt gas mileage or power.

Solution: I think it is the throttle position sensor, but I want to make sure.  Is there some sort of test?  What else could the problem be?  Is there a way to take it (the TPS) apart and clean it (assuming somehow the sensor has gotten dirty or fogged)?
If not, is there a place to get one cheaper than the Audi dealer quote of $170?



TIA
Joel Liggins
'90 V8
'86 5kcstq
both with Konis; more upgrades to follow