Overheat with high water pressure

My 92 Maristar 240SC 454 seems to have two lives. Sometimes it runs fine and the temp and water pressure are normal. After it sits, it may run hot (~200) just at the alarm limit, and the water pressure just about maxes out. The next time I drive it they may go back to normal.

These seems to be some obstruction limiting water flow through the engine, but not sure where to look. I don't know where the pressure sensor is, but the blockage must be after that.

Any ideas?

Thanks