Thanks to Amarin, Dave and Nahtanha for you responses and suggestions.

I swapped cars with him yesterday afternoon. After replacing the gray cap with a black one I drove the 20 miles home. There was just a bit of coolant on the reservoir at the end of the drive. It may have been there prior to switching the caps.

Later that evening I removed the reservoir cap and allowed the car to idle for 5 minutes which brought the temp gauge up to normal range. There was just a bit of increase in the coolant level that might be expected as it heated. No agitation of the coolant in the reservoir.

I installed a new 192 thermostat this morning. I let the car idle for 5 minutes with the cap off with the same results as yesterday. I then raised the rate to 2500 rpm for an additional 3 minutes. The only change in the coolant reservoir was the final level was just a bit higher than where it had been after the 5 minute idle. I drove it this afternoon and so far the cooling system is not showing signs of excessive pressure.

Perhaps the base of the problem was a defective gray reservoir cap? So far it seems to fine.
--
Any twenty minute job is just a broken bolt away from a three day ordeal