I just fixed my 89 that was doing the same thing. Turned out to be the check valve in the fuel pump.. Put a fuel pressure guage on the fuel rail, jumper the diag connector to activate the fuel pump. You should then get 40 psi, turn key off, the pressure should stay for 3 min.. If not disconnect return line, plug the fuel rail return line and retest. If it still drops, FP /check valve, if ok then Pressure regulator..