2000 Pontiac Firebird Car Shakes, Surges and Dies when idl

I purchased my Firebird one month ago. Since then, the car has died on me 6 times. Each time the car has been idling (either while in "park" or "drive" at a stoplight). The car shakes, surges, and dies. My oil guage and battery guage needles go back and forth during the shaking and surging. About half of the time it starts right back up. The other half it takes 3-4 attempts before it will stay started. It has been hooked up to diagnostics twice. The first time (after dying 3 times) the computer registered a trouble code for high voltage on MAP. The technician reset the car's computer. I took it back after dying again and no trouble codes registered.

I phoned the previous owner and he stated that it was doing the same thing to him. He said it might go for 2-3 months before dying. He stated that he took it to the dealership 3 times and it never registered a trouble code.

Any ideas?

Updated 02/28/2008:

The car stalled out when I was driving approximately 30 mph. No warning other than losing acceleration. This is the first time it has stalled while in motion.

Going back to mechanic. I'm thinking what if a trouble code doesn't register, as has happened previously with this car. Any ideas?

Updated 03/01/2008:

Once again no trouble codes registered. Additionally, it stalled 3 more times (once while driving) since the 02/28/2008 update. Please help.

Updated 03/25/2008:

Replaced MAP sensor and crank sensor. Still surges and stalls out, but only when gas tank is less than half full. When I fill the tank up, it runs fine until I get it down to under a half tank. Could it be fuel pump/relay? I'm ready to give up. Any ideas?