AC works when it wants to

My 1986 300E AC unit did not work. The systems is fully charged and the service tech checked for voltage getting to the clutch, found none. He then by-passed everything and went directly from the battery and the clutch still did not engage. I was ready to order a clutch when the systems started working, blowing cold. Worked great for three days and then stoped again. Any ideas?

Thanks in advance

Mick
I love my toys

86 300E
89 560SL
89 Bronco II