Simultaneous overheating and loss of A/C

[SVX ROO]

Re: '94 SVX with 207K miles

I am struggling with a dual problem that occurs at same time. It only occurs after a steady drive at high speed but eventually in stop and go traffic which I am seldom in. The air is blowing cold and fairly quickly is no longer cooling. It seems lowering the temperature setting a degree or two gets it working again but only for a couple of minutes. By the time I have fiddled with the air controls I will see that the temperature indication has gone up. The temperature goes about half way from normal (about center) to the red line. It never makes it to red and there has never been steam. As long as the temperature is high the A/C does not work. If the temperature drops, the A/C will work again.

After the first occurrence, I replaced the thermostat and radiator cap. I completely purged the fluid and replace with new A-freeze mixture. The radiator and thermostat housing look very clean. There are no leaks: external, to oil or to compression. Both fans are running when there are problems.

I have now found a bit more of a strange set of circumstances. Once the problem has started, if I drive in third gear (set at 3) rather than drive, it will cool down and A/C will work. I went through 3 complete cycles of going to 'D' and back to '3' while driving at about 55 mph. Each time in D the A/C would quit after a minute or two and then the temperature would go up. Go the 3 and all would go back to normal. Now I can understand that a bit higher RPM will spin the A/C compressor faster and also the water pump, but this still seems very strange.

I have two specific questions:
1. Is there a temperature switch somewhere in the cooling system that shuts off the A/C compressor.
2. Has anyone experienced loss of water pump efficiency on a high mileage SVX or other Subaru? (Water pumps usually get noisy, leak or seize, but I have never seen one that pumps less than it should.)

Answers please or any other ideas.
Ken