For 23 years, the wait to start light goes out after a few moments, then start the diesel. BUT NOW, Intermittently, the "wait to start" light goes out for a half second, then re illuminates; cycles over and over. It does it when the engine is warm, after having been run recently. This is new. Might this just be a bad relay for the heat matrix or is my heat matrix finally going? '93 W250 1st gen.

 

Probably a bad Air Intake Temp sensor. Disconnect it and see if the problem goes away.

 

Is the Grid heater cycling too of is it just the light? I agree with TC if it is cycling the heater. I would also check and clean all the ground wires. If just the light is going on and off the PCM may be at fault.