First time owning a legendary Supra really could use some help.

[Lopt]

Hi everyone who reads this,

I hate to bother people but I really want to figure out this problem before it turns into something worse. Just bought a 1988 Toyota Supra non-turbo, rescued it from rotting.. it drives great but has just started to have some weird issue with the temp gauge.

It fluctuates rapidly, happens like this: I warm the car up in the warming, get it to it's normal below medium temp and take off for work.. about 10 minutes into the drive the needle will rocket up to the H for five or more seconds then shoot back down to middle or below mid... then right back up, repeating this process throughout the drive.

The oil isn't milky and is still clean.. no bubbles, the cap did have some steam coming from it when I parked it this afternoon, I let it idle and it stayed hot. I'm going to replace the thermostat tonight to see if it was just stuck.. but.. I'm having doubts there as well.

I'm really confused and could use some help,

Thanks.