I have the same problem, and it sucks when I have to drive it in the cold. Feet freeze and seems there is nothing I can do. I even replaced my DDC and that didn't help. For some reason I always thought the carpet got unhooks or something.
welll...,either a vacuum line came unhooked or broke, or you have two blend doors that are bad...from what I have gathered anyway.
98 GTP Coupe- 180* T-stat, GMPP Front Sway Bar, Dual Aeroforce Interceptor's, TDC FWI, TDC Taylor 10.4's, ZZP SS Plog, ZZP 3" SS catted DP, ZZP MPS W/3.4, AL104's gapped @.055, 3800Perf. PCM