cooling
 

I have a stock 86 GT that at idle runs cool, but in normal driving it runs almost too hot. I have heard that aftermarket, not OEM, temperature sending units are sometimes the culprit. It has a new water pump, fan clutch, and radiator. I am also looking for a good emissions legal cam that makes more power than the E303.
My other Mustang that needs the cam has a 352 cubic inch stroker with 10.5:1 compression. Fox Lake Trick Flow heads and intake that are Stage 1 ported.

Sounds like the temp guage/sensor are screwing with you. My car runs cooler while cruising than it does at idle. The guages have been known to do some wack stuff though. When I turn on my lights, the car runs like 80 degrees hotter;)

check the obvious stuff first. Alot of people use teflon tape when they install sending units and Ive heard this can be a problem because the sending unit needs a good ground to work correctly... so if the threads dont contact the block well it can mess up the gauge reading.

The other parts you say you replaced. generaly overheating while driving is caused by not enough circulation. So a bad water pump could be the colprit.. or thermostat... the two things that control the flow of coolant.. though it could be a clogged up rad... but like you said, that's new, so I doubt it.

overheating at IDLE is usualy a FAN issue... so maybe teh clutch is shot or there's no shroud... though ov course, a water pump or thermostat could cause the problem there too..

If you're lucky, it might just be the gauge.. though I dont see why it would give certain reading based on idle and driving .... specilay if it's consistant, if it were just a goofy gauge it would be all over all teh time... but I could be wrong
-as

lemme add.. before you go nuts... grab a new sending unit and try it... they cost like 5 bucks... it'll save you alot of hassle if that's the simple solution. I've had bad sending units.. when I bought my car the gauge didnt move at all... so I put in a new sending unit... and then it was fine... though after a while it wouldnt go past the 1/4 mark it would start at the bottom but wouldnt go farther then that mark. So I grabbed one for 3.95 at autozone and it worked flawlessly.
-as