[sawdust]

I have a 1991 Mustang 2.3l dual plug, 5spd.

All last summer and up till a couple weeks ago the temp never went above "A" in "Normal" (or second bar from the bottom). Lately, while driving, it will go up to "R" & "M". But, at a stop, I can tell that the fan is on and the temp settles down between "M" & "A" again.
In the past 6 months I have replaced the water pump, and the temp sensor (located on the upper heater core hose). Last month I replaced the ECM, Coil packs and the battery cables. But these wernt replaced as a result of this current temperature issue.

The radiator is full, resivour is at the propper levels (& always has been) & the fan seems to be working as it should be.

Any suggestions?

[evintho]

Sounds to me like your cooling system is partially clogged. I would give it a thorough flushing.

[FrankBullitt05]

Reverse flush the cooling system and see if that fixes it