a/c compressor only engages when engine is under load, for example, uphill and over 3000 rpm

I have a 1993 Buick Park Avenue, 3.8, 4 speed automatic. The a/c compressor will
come on, but usually only when there is a load on the engine, for example, going uphill,
rpms go over 3000, and I get cold air. On the downhill side, or at idle, the a/c clutch
does not seem to engage. Mechanic says I am a little low on Freon, but that even
when jumpering the power and the ground, clutch does not engage.Thanks for your
help. I don't want to put $700 for a new compressor into a $700 car!