Overheating?

[WarrenG]

My 289FIA has approx 1900 miles on it. Equiped with 1965 289 stroked to 331. Last night on my way home after driving approximately 25 miles and 40 minutes I thought I caught a whiff of coolant. When I looked at my temp gauge, it was up to 240-245 degrees F. After approximately 1-2 minutes it quickly dropped back to its normal 180. Approximately 15 minutes later while accellerating from a stop light, it moved back up to about 225-230, then within a few seconds quickly dropped to 180 again. This happened three more times with the top out at between 210 and 225 degrees. With the exception of the first episode, each time the peak never lasted for more than about 30-45 seconds and the gauge returned to normal operating temperature in less than 1 minute. For the last 38 miles and 45 minutes of the trip, mostly on the capital beltway, the temperature remained normal.
Other than the water temp everything else was normal. Oil temp remained between 175 and 185 degrees and pressure at 45-55 psi depending upon RPM. It was about 10PM and the air temp was probably around 70.
Upon shutdown, I checked the engine compartment but didn't find any sign of leakage however there was only about 1/4-3/8 of coolant in the Hamilton overflow tank.
Any ideas what might have caused this before I start taking things apart? My first guess was faulty thermostat, but was wondering if a faulty sensor, gauge or wiring might also cause the indicated fluctuations. Any help here would be appreciated.