I read that you checked for vacuum leaks, but have you actually checked the vacuum to see if it might be the problem? My car did the exact same thing when I had the vacuum leak.
Also, I've heard the ECT sensor may cause similar problems. The ECT sensor is on the driver's side near the front on the lower intake. I believe they sell for around $15 at the Auto Zone near me.
Hope this helps. Good luck.
------------------
Gunning for 12's!
Buy your parts here
ICQ# 42269241