Air Conditioner won't stay engaged.

I have a 1984 GMC Suburban Sierra Custon with 130,000 miles. It has the 6.2 Liter Diesel engine.
The problem I have is that the clutch on the air conditioner compressor engages for a few seconds then disengages. It continues this cycle but doesn't stay engaged long enough to begin to cool.

Thanks,

jbwolf