temp gauge/ overflow issue

[Wilson_]

Here is the deal, I work swing shift and go to work around 230pm and leave work at midnight.

On the way to work truck runs fine temp gauge (auto meter sport comp) stays at 180.

on the way home the truck runs fine but about 5-10 mins into the 20 min drive home the gauge pegs at 250 very fast, then drops down to 200 then a little slower pegs again then drops to 180 for the rest of the way home. I do loose coolant about a 1/4 of a gallon or so that spills out.

The probe for the temp gauge is in-between the 6 and 8 cly in the head. Should I move it to the front of the intake mani?

Here is what I have tried.
New thermostat and housing. ( the old one was pointing straight up and was off of a tbi motor)

New 16psi Radiator cap. Truck has a 87 Trans am Radiator in it. should it be higher than that??

I tried burping the system with the front of the truck pointed uphill with the rad cap off and running.


Also the truck has no heater and all the holes have been plugged

Any ideas??

Thanks