After running the A/C (which otherwise cools great) for 1.5 hours or so, it starts to blow warm air. After I turn off the A/C for 15 minutes or so, I again get cool air. I interpret this behavior as the evaporator freezing up and then thawing.
I thought I would check the adjustment on the clutch cycling switch to make sure it wasn't too low (which can cause the evaporator to freeze), when low and behold, there is no adjustment screw on my switch! I removed the switch from the accumulator and it does have a Volvo P/N on it. Does anyone know if the 93s were the only years that didn't have a screw adjustment on the switch? Can I use an R12 switch from an earlier car and then adjust it for R134a?

My car does have a temperature shutoff switch on the compressor so that could be the cause, but I doubt it as this problem only occurs when cruising down the highway for a long stretch - not in stop ang go traffic which I would think would cause the system pressure to be greater. But, if anyone thinks otherwise, I'd appreciate hearing about it. Thanks.