Buick/Olds: 91 Olds Custom Cruiser, electronic climate control, vacuum lines

Question
I recently purchased a 91 Olds Custom Cruiser. Did the conversion from R12 to R134A. When you drive the car with the AC on it will every once in awhile blow hot air and then in about a minute or two start cooling again. I noticed it doing it more when passing or being passed on the interstate. If you adjust the temperature on the electronic climate control like 2 degrees for example 67 to 70 it kicks out really hot air. What could the problem be?

Answer
This is most likely either caused by low vacuum or low refrigerant...however, the question is, "why does the engine have low vacuum" if this is the case? 1. Check to see if all the vacuum lines are on and not leaking. 2. Check to see if your a/c system if "full" or perhaps a bit low on R134A. If you have guages, check the high and low side pressures. If not, check the sight glass in the dryer, if you see air bubbles, that is your problem...
Did you do the conversion yourself? If so, did you change the expansion valve, and evacuate the system, before adding the polymer oil and the R134A?