got a 2000 GMC with 6.0 the air conditioner goes on and off I thought it was going out on low pressure, pulled freon out and replaced filter orfice and wieghed charge in works fine until I drive and found it is still going out, checked and found no problems with high or low pressur switch, I thought I had found problem when I rev up and it started working agian I thought it had something to do with vacum but when Pulled control panel apart I found no vacum hoses I still think it may be in control panel but im not sure, it is not low or high pressure switch or a/c relay, is there a sensor that senses cond. temp, is the a/c controled by computer im at a loss

I am far from an expert but I do have several A/C certs. You have a WOT (wide open throttle) relay that could be faulty. I am not sure where it is on the truck but it senses that you are asking the truck for all the power the truck can give and it shuts down the a/c till you let off the gas. Also you might have a clogged receiver dryer... usually when they are bad or clogged you need to be moving or under throttle to get the a/c cold.
Keep us posted on the results.

I will look for relay and keep you posted, that makes sense to what it is doing