Switches re Air conditioning.

Have a 1999 GMC Sierra 1500 w/ 5.03 engine. We have been having problems with AC starting to blow warm air, we have to shut it off a few minutes and restart and it will usually start to blow cool again. This started last year and we had several places work on it but nothing has changed. after replacing a lot parts that pertain to AC but nothing has changed.,  I have been noticing that if you set the fan speed on 3 or 4 unless you move the speed dial down the air temp starts to get warm again and if you turn the fan off then after a minute or so and turn the fan back to 3 or 4 the AC will start cooling again even though the temp setting or on off AC switch or temp setting dial has not changed in anyway.  The compressor has been checked several times and every one has said it is working fine and is not the compressor but that is the only thing that has not been replaced.  In my mind if the compressor is functioning is still functioning 2 years that we have been experiencing this problem it would not check out as OK.  We have spent over a thousand dollars in parts alone and changed nothing.
Three of my sons are mechanics by trade and they are stumped.
I am looking for someone who has delt with this  as we are running out of answers for our vehicle and  a friend who has a identical truck, make, model and year as ours and has the same problem.  Every body wants between 1500 and 2500 to "fix " it but no one that has  worked on it hasn't made it any different.  We are seniors and do not have a large income to keep putting money out for repairs that are obvously not the problem