2002 Envoy a/c intermittent

Occassionally during very warm weather my a/c doesn't work, this usually is cleared if I turn the engine off then back on. I've brought this to the dealer on numerous occassions, without any resolution. Does anyone have any idea what the problem is?
:banghead:

Several issues with tsb's written, one refers to a bad ac low pressure switch being erratic, but there are many other possibilities, they must me checked with the ac inop for a one time accurate diagnosis.