Kevjuice

The Air Conditioner in my Honda has always been pretty cold, but recently I was driving down the highway and the A/C began to blow warm air. The air stays warm for about 2 minutes, then it cycles back to cold. This process recently repeated several times on a drive down the highway on a day when the temperature outside was around 101. I had a mechanic look at it, he hooked up a the low side and said that at idle the pressure was about 40, but when it was revved up to higher RPMS, maybe 3000-4000 the pressure was about 20. Is that normal? Should that happen at higher RPM, or could this be the cause of the warm air? I had the compressor and dryer replaced about a year ago at the dealership. Any recommendations?