History:

Sporadic warm running with A/C.

It will run perfect sometimes, other times I look at my guage and it is quickly approaching the red. Off with the AC and turn
the heat on, temp goes down. Again, this does not happen all the time.

I am leaning toward the fan relay, but would it work sometimes and sometimes not?

Result: Replaced The TStat, problem solved so far, that was 5 days ago, temp guage dead center now.

Thanks for the help, Art

The Chris Herbst suggestion is to pull the connector off the high pressure switch (passenger side frame rail, near headlight), and jumper across it. This will make the pusher fan go on whenever the AC is on, which offers some cooling benefit.

However, I just had a similar situation with my '92, and previously with my '88. In both cases it was because of marginal cooling with the radiator. You might try cleaning the fins out with a garden hose or compressed air - remove the fan shroud, and blow from back to front. It's easier if you remove the radiator because you can instantly see how much crud gets blasted out. If you're lucky, that will make all the difference. But the AC does tax the cooling system, and a 12-year old rad may not be up to the task. I just replaced mine, and I'm happy to say that even in 116 degree desert heat over the weekend with AC on, cooling was just fine.