Have 97 CV P71 the ac compressor comes on and goes off, it acts like one would without the proper amount of gas in the system however a/c contractor put the gauges on and stated that it has plenty of R134A in it. bad relay?

 

bad pressure sensor or a blockage in a line…
you need the gauges hooked up to watch pressures when the compressor tries to operate...

 

The A/C systems as designed are supposed to cycle on & off. The timing of which is affected by things like ambient temp, engine RPM, cool setting to name a few.
If there is any moisture in the system it could be freezing at the orifice tube and causing the low press switch to open. It would be a good idea to do a little research on how the system is supposed to operate and some of the possible causes for what may or may not be a problem.