ac works fine at speed for about 100-125 miles, then just stops putting out cool air. Turn it off for about 15 min, then back on, then it works for awhile longer. Also when I come to a stop, the ac unit stops cooling. My quest is, is there a ac relay that is not doing its job? Mayby heating up and not making a contact to keep the compresser on?

Strange. It sounds like the compressor is going out, but the performance should drop off sooner then 100 miles. Check the condenser to see if it's blocked with dirt. If you have not had the freon replaced with R134A, run the AC and check for bubbles at the sight glass on the dryer.

low charge? sounds like the evap maybe icing up