Hello stratus,
Glow plug temperature inhibit is controlled by the Electronic Control Module(computer) which receives temperature information from the coolant sensor,located in the water crossover on the engine.The computer sends a signal to the cold advance relay in the ignition circuit to the glow plug controller.The relay is located at the junction block in the engine compartment on the right side of the cowl.Make sure your temperature sensor is not reading incorrectly & this circuit is good.Sometimes a sensor will still work but the values will be "shifted".
For an accurate test ,this must be done when the problem occurs.
Master A.S.E Automotive technician
A.S.E MasterTechnician A1-A8 w/L1,38 years experience General Motors Master Technician