My '85 L- Jet spider stalls after starting in about 30 seconds. RPM drop to about 600 and the engine dies. Sometimes the RPM will drop to about 600 and then increase to 1200, but then goes back down to about 600. Car usually stalls. Full temp idel is about 1,000 rpm. Car runs fine when at operating temp., perhaps a little rich. Swaped aux. air valve and replace temp. sensor to no avail. Any suggestions?