I have a ’69 Mustang with a 302. I've had this car for about 5 years and had the engine rebuilt about 3 years ago. The rebuild included a new radiator, water pump,and electric fan (in addition to the engine driven fan).

There is one problem that I've always had with it. When the engine is fully warmed up and I'm sitting in traffic the temp gauge will sometimes climb all the way up to the line just before the H on the gauge, hold there for about 10-20 seconds, and then go back down to normal. (It normally holds steady just over the left leg of the "M" in the word "TEMP" on the gauge) The movement of the needle from normal to high, and vice-versa, occurs in the space of about 3-4 seconds.

I've replaced the thermostat several times, and the sending unit once; both to no avail. I don't think the engine is actually overheating as I tend to think that the quick movement of the needle would indicate otherwise; in addition the engine runs fine. There is no burned oil, or other signs of overheating.

Does anyone know what may be going on here? Are temperature gauges prone to erratic behavior like this?