A C182 Pilot reported problems with his pitot-static system that resulted in erroneous airspeed and altimeter readings; which led to an altitude deviation. When he selected alternate static; all instruments returned to normal indications.

Narrative

I was at 9000 FT; in and out of the clouds. I was asked by ATC to come left 10 degrees and descend to 7000 FT for traffic. This was later amended to 8000 FT. I reduced power and configured for descent. My airspeed indicator approached red line (VNE) but the VSI indicated 0 FPM descent. Altitude remained unchanged at 9000 FT I reduced power further; ultimately to 13 inches MP. Still at red line; 0 FPM descent and 9000 FT on altimeter. I crosschecked with the GPS and groundspeed was 120 KTS while the airspeed indicated 180 KTS. I realized something was wrong with the pitot-static system and activated the alternate static. The ASI immediately read 100 KTS; the VSI 750 FPM descent and the altimeter 7500 FT; which was 500 FT below my assigned altitude. I immediately climbed back to 8000 FT. After rechecking the aircraft systems; I requested a descent to VFR conditions and was cleared to 7000 FT. I was handed off to Approach and preceded to my destination under visual conditions. The aircraft was taken for repairs the next day and the pitot-static system cleaned and tested.