Problems with A/C

I'm working on a 1990 D-150 318 truck. Let me give you a history of what i have done. Today I pulled the freon out of the system and found it over charged. The truck originally held 44 oz. of R-12, it had been converted to R-134 before I got it. Today I installed a new dryer, vacuumed the system and charged it with 80% of the R-12 charge, 35 oz. of R-134. Now the system cools down great but I'm having a problem with the comp. kicking out prematurely. My gauge readings are good 25 low side 200 on the high. I put a test light on the comp. hot wire to see if the clutch coil might be weak, but that is not the case. Vent temp. blows to about 55 degrees but then the comp kicks out. And it seems to not be due to temp. it kicks out at different times. I didn't get to jump the low pressure switch to see if that is the problem. Does anyone have a electrical schematic of the electrical side of my A/C system?