i have an 89 firebird formula with the tpi injection it will run fine drive down the road for about 30 to 40 mins and then shut down its getting fire but losing injector pulse what would cause this i have replaced the computer and the distributor and the fuel pump

When it does it see if it is losing power or ground with a test light. The ECM/computer supplies the ground and the ignition switch supplies the power to the injectors. See if there is power on one wire and ground on the other.If you have power or ground on both wires there is a short in the harness usually in a connector. No ground bad ECM - No power start at the ignition switch.