Toyota Supra fluctuating temperature gauge

Temperature gauge in my 1988 Toyota Supra fluctuates pretty erratically. I replaced the thermostat (boiled it and it opens and closes) and the coolant temperature sensor. I replaced the radiator about a year ago. I drained the old fluid, installed the new thermostat and filled it up with new fluid.

I will drive around for awhile and the temperature gauge will sit below halfway and after awhile it will move upward toward H in a matter of seconds. Hitting around H, it will will drop back down about the middle of the gauge and then move back up toward H in a matter of a few seconds. Sometimes I can tap on the gauge cluster covering it and will bring it back down on certain occasions. The reservoir tends to overfill after I drive around for a bit and then it spills out onto the road.

I opened the hood of the car after it has neared H and even on H and it seems normal just as if its been running around the mid-line of the gauge.

I was just wondering if it could be a bad temperature sender, bad temperature gauge in my car, or a bad radiator cap. I checked my oil and did not see any radiator fluid present on the dip stick nor did I see any oil in my radiator fluid.