OK, K.I.S.S. When I first got the truck it had this issue. I immediately thought it was a flow problem so I replaced all the aforementioned stuff and still had the problem..
Day before yesterday, I bought a new thermostat housing that had bosses for an aftermarket temp gauge. While the housing was off I bought a 195 degree Stant Superstat to replace the MotoRad 195 that was in there from 5 months ago.
So, a new problem started, it ran at 205 constantly(new gauge reading) and on the R(dash gauge). When romped, 10-80mph run, It would jump up to 220ish and was in between M and A.

At this point I realized how much that different thermostat had affected the issue and immediately thought about flow restrictions again. SO, I went to my ole standby that I was running in my 95 F150 and every Mustang I have owned since 1992 including a 1989 5.0 Notch, and 2-1989 5.0 Hatchbacks... And that was a High flow Thermostat(use to be Robertshaw now they are China Mr Gaskets).

So, far it's been 83 degrees today and I sat in the driveway running the A/C at a standstill for about 15 minutes then hopped in the truck drove about 100 yards then did a 20-80 mph run. Then drove back roads at 45mph for 3 miles with A/C on then did a 0-95mph run. Then 1 mile at 55 then into my driveway where I immediately turned on the A/C and let it run for 5 minutes.. The temperature never rose more than 205 (Beginning of "R")and during the hard, acceleration runs it never went above 200 (top of "O")...

I Think its cured.. I will update if something ugly surfaces but it seems the two other thermostats I ran previously were restrictive during higher RPM driving. It could be that I have been running HighFlows for so long I didn't know what to think when I ran a regular Thermostat and I seen that gauge jump up to the M/A range it was foreign to me

 

Congrats, better flowing thermostat.


I would not have figured the other as bad, but appears that really wasn't designed for the motor's application.


The plus is now you have an actual gauge.

 

Yea, actually I added a pillar pod with an oil pressure gauge and most importantly a Trans temperature gauge alongside the Water temp.. Should make things a little bit easier to keep an eye on when towing our RV..