a/c compressor cycles on and off

I have a 1988 Pontiac 6000. A/c compressor does turn on sporadically. May stay on awhile, other times cycles on and off.
Checked hose into evaporator, is cold when compressor on.
Checked low side pressure, 33 psi when compressor on, rises to 65 psi when compressor off.
What is normal Hi\Lo pressure when system full of R12?
Thank you.