92 caravan cooling problem

I have a problem with 92 caravan. I had leaky radiator back in 10/2003 changed it out car ran fine late in 11/2003 I had problems with the car over heating replaced the thermostat still had the problem removed the thermostat car ran fine little cold to drive in but all was fine. Put in a third thermostat seems to work fine. But now I have an under heating problem. The coolant is at 190deg when the car is idle. After driving the car about 2 miles or more I can see the temp gauge cycling down and then back up to normal temp. Pulled over during one of the down cycles and got a temp read from the coolant of 160. No more over heating and the heater seems to blow out plenty of hot air just have the cycling thing with the temp. The old radiator was a single wall and the replacement was a double wall. I also replaced the cap which doesn't seem to seal all that well I get back flow into the reservoir even when the car is at 190. Which in turn I have no pressure in the coolant system. I put the old cap back on and the system seems to maintain pressure. Any idea why I would be seeing this large temp difference in the coolant 30deg change.

possible the water pump is not cycling properly