Sporatic temperature guage

Fellow members,

I have a 1995 Ford f150 5.8 engine with a sporatic temperature guage flucutation. The original cluster temperature is broken but has a Supro analog add on guage. I have changed the following parts consisting of Thermostat, Water Pump, Radiator, fan clutch which was defective, Radiator cap. I will be changing the temperature sender today possibly. My probem is when I take the vehicle up to highway speeds the temperature guage will go up to about 240 degrees for a few minutes and then will fall back to normal temperature range according to guage and seems to be alright for a while. Any help will be appreciated.

a common thing that might cause this is a engine to body ground check that .there should be a large braided ribbon on the back of moter or buy the moter mount