Sounds like it could be you MAP sensor. It's a little black thing on the right hand side of the firewall with wires and a vacuum line attached. Check the connections and see if it helps, if not, try and get a hold of one and swap it over and see if it fixes the problem.
1992 AXII Telstar TX5 Ghia
121kW 2.5L DOHC V6 in a car that weighs 1100kg's and looks like a 4 banger from the outside......SLEEPER!
1988 EA S Pak 3spd CFI.....Soon to be a 250rwkw HSV eating machine.