Owen_Dyneto wrote:
Unless you know it's been done recently, on a new acquisition I'd always recommend pulling the water pump and distribution tube, inspect them, and pull a welch plug or two and flush out. But maybe first you should use a good laboratory quality glass/mercury thermometer (forget the roast beef one in the kitchen) and insert it in the radiator neck and compare that reading to the position of the gauge.

If you browse this site you'll MANY inquiries similar to yours and some of them gave very good comprehensive advice and lists of things to check; you should find and review them but here's a few things to consider:

1. Retarded ignition timing
2. Lean carburetor mixture
3. Undersize exhaust or tailpipe
4. Restrictive muffler
5. Too narrow a fan belt (rides on pulley base, not sides)
6. Blown headgasket (do a compression test)
7. Collapsing or deteriorated radiator hose, esp lower.
8. Excessive distance between fan and radiator