I'm sure you've checked to make there is plenty of R-12 in the system, Compressor will come on even if there is no R-12, due to not having a low pressure switch in-line with the clutch electrical lead.
If suction hose is sweating all the way to the compressor, then I'd say you've got plenty of R-12. Otherwise, you might have a stuck POA valve at the outlet of evap. This was very common, allowed the compressor to cycle on&off in short cycle times, less than 2-5 seconds due tothe compressor thermal switch.
Next I'd look is to remove glove box and glove box. Move TEMP knob and see if door is really moving. There is a pinch bolt for the cable housing, it may need loosen, adjust cable for max cold, move cable to make sure door is closing all the way. Monitor air temp at vent(s). Check other cable operations, too.
CarlC gave very good tips, as well as the rest did.