It sounds like trapped air to me also, since it used to happen to me.

What I did was replace the thermostat. The replacement thermostat had a tiny hole and valve thing on it. I just snapped off the valve thing, leaving the tiny hole clear. No more temp excusions.

Now, the only time the temp gauge climbs is when it IS hot outside, like driving up out of Death Valley to Beatty, NV and it's 127 degrees outside. That's hot.

Good luck.

Al

KI4Dxx
?85 Toy 4Runner: 35x12.50 BFG MTs, manual, Kenwood 2M, 140 amp alt, 26 gal aux fuel tank
'06 F-350 SRW 6.0 PSD XLT 4x4, manual, Yaesu 2m, 140 amp alt, 90 gal aux fuel tank, Air Ride air bags