Could be several things....check a couple of these out. First...when it is blowing hot, make sure the a/c compressor is running. Do this by taking a look at the very front of the compressor and if any part is sitting still, then that's the problem (sorry if you already knew that part), then check out these parts to see if they are okay...well, I'll have to update this post with the link in a minute.
Edit: check out this link to make sure you don't have this problem:
http://www.kronowit.com/porsches/944-heater-fix.html