'92 300SD A/C problem
 

I've got a 1992 300SD (W140) and I love it—despite all that you have heard about it.

However…I'm currently experiencing intermittent A/C operation. Here are the symptoms

1. Occasionally goes to full heat in Automatic mode. Pressing the Defrost button twice, to cycle through Defrost, often clears the problem.
2. Displays error codes 74 and 75.
3. Seems not to want to A/C at temps somewhere above 75°F. I can't really check this out as global warming is in effect here and we haven't had all that many days when the temp has been that high. It doesn't seem to heat in this failure mode, just fresh air.
4. Sometimes goes to full heat mode after the engine has been cycled several times (as when running errands). Defrost trick fixes it normally.
5. A/C runs well at idle and at speed in lower ambient temps (50-70°F)

There's a DIY article which suggests to the nimble minded that this is similar to circulation pump failure as do the codes. My question is "Does this all fit the failure of the circulation pump pattern?" And, if it is the circulation pump, where is same? Is it also called an auxillary water pump?