68 overheats. Why???

First, is it really getting hot? or is the gauge messing with your head?
I used to have a '66 Coupe so 2 things come to mind. Check/replace the temp sending unit.
The second thing is a small silver instrument panel regulator that is mounted on the rear of the gauge cluster. It's about an 1" long and 3/8" wide. It just simply unplugs. If this little regulator is bad it will give you weird readings on various gauges in the panel.

Thats a couple things to check anyway. A good shop manual on this car will be indespensible.

Good Luck, GT

i would first put a good mechanical gauge on first and see if it is really running hot. as stated the IVR could be going bad, or your factory gauge might be out of whack. next time you see the temp going up, check the oil pressure gauge and the fuel gauge as well and see if they are going in the opposite direction. my 66 had an IVR malfunction years ago, the temp gauge would peg at the top, and the oil pressure gauge would peg at the bottom. they would return to the proper reading after a few seconds though, but once i changed the IVR that just stopped.