A/C Problem 99 Suburban wont cycle off

I have a good deal of automotive knowledge, though not a professional tech, and living in Phoenix I have diagnosed and repaired a lot of A/C problems, but this one has me baffled. When I fired up my A/C for the first time this year, I discovered that my clutch was never cycling off. It disengaged properly when the A/C was turned off, and when the clutch cycling switch was unplugged, so I assumed that the relay was OK. Even though the symptoms aren't correct for this problem, I replaced the clutch cycling switch, as it was easy and inexpensive, but no dice. The duct temp seems correct at idle for Phoenix, but I am afraid to road test with compressor always running. Is there another switch in this system (perhaps a "High Pressure Switch", I see a switch on the hi side of the manifold) that could be causing this. Could it be undercharged (my experience tells me that an undercharged system generally cycles on and off rapidly)? If I dont figure this out soon, my wife and kids are going to kill me in my sleep....HELP!!!