A/C and idle questions

Today the A/C in my 91 PA decided to go in economy mode. It wouldn't go in to 'auto' any more... I ran the diagnostic mode but there were no errors beside the old 38 code. When I turned on the engine again the 'auto' mode worked again and kept working, but ambient temperatures have dropped 20 degrees.. does this have any influence? Why would it go to economy mode?

Also with half warm engine the idle is a bit unstable going up and down fast... I suspect this has something to do with EGR or PVC valves, I have never checked those.
Does the 91 PA have a digital or conventional EGR valve? Are there any other things beside ignition that I can check?

If you are low on refrigerant, the system will go into econ mode. In addition, if the system is low, the clutch will cycle off/on resulting in idle fluctuation. Has your A/C system been converted to R134a?

No, it is R12. It was recharged earlier this year. I checked the clutch a couple of days ago when I restarted the engine, it was not cycling. It was over 90 degrees then...
Today I had the A/C on auto for over an hour. It was cycling but kept on blowing cool air on auto, but the temperature was 70 degrees at most which would be normal at lower temperatures right?

I could not notice anything while idling...
Is it correct that the 91 base model doesn't have a EGR valve? Haynes states it has, but I really can't find it...