AC problems?

VIN #JALC4B14527000163

Having some problems with this customer's A/C and wanted someone's input. I replaced the condenser fan and system dryer using OEM parts in May. A/C was working fine the past two weeks until the outside temps started climbing; I'm in North Charleston, SC and have had temps in the 90's & near 100 the past couple weeks. I have the system charged to good low/high side pressures (no sight glass) and with approximately 44 deg. Vent temp at 2000 rpm. Twice in the past two weeks the driver has been on a hour and a half round trip and on the return leg the dryer has blown off pressure from the system and it loses r134a and coolness. I charged it back up the first time and checked and didn't see any problems. The second time I charged it it was showing the high side reading up around 250 psi so I didn't charge it any more. Two days later the driver told me the same thing had happened again, in the same way and on the same route; it never happens on any other route. I checked the system yesterday and recharged with approx. 1/2 lb. Of 134a. At idle the compressor engaged readings were 23 psi low/165 psi high & compressor disengaged readings of 40 psi low/165 psi high. At 2000 rpm the comp engaged readings were 20 psi low/210 psi high & comp disengaged readings 40 psi low/165 psi high. The compressor cycle time at idle was approx. 15 seconds engaged/10 sec disengaged; the cycle time at 2000 rpm was 20-25 sec engaged/10-15 sec disengaged. I was thinking earlier that I had a high side restriction somewhere that was causing the HS pressures to spike under certain conditions but the readings I got yesterday don't indicate that. Would like to know the system operating pressures for this vehicle and get any suggestions you may have; dealer hasn't been very helpful with this.

To my knowledge the cond. Fan only has one speed and I think it comes on around 150-170 psi high side; can't remember the pressures for certain. There is a pressure switch on the drier; the original OEM drier also had a sight glass but the replacement OEM drier I installed doesn't. The sister truck to this one has a sight glass in-line just before the high side connector port.

Thanks, didn't know about the two speeds. Do you have anything that shows what the pressures should be? I won't see the truck till next week but will definitely check this and would like a comparison on the pressures.