I have a 2001 Chevy Silverado 1500 4X4 4.8 Liter AC issues....

[awbooth]

I have a 2001 Chevy Silverado 1500 4X4 4.8 Liter.
I have replaced every component of the Air Conditioning System over the past few years and the system has been working very well.
This year however something different is happening with the AC System.
I turn on the AC, make sure the manual control knob is in the cold position, make sure the recycle button is pushed, made sure the AC button is pushed in. 
The AC doesn't work right away, warm air comes out of the vents. Then when I shut off the system and turn it back on it comes out cold- around 45 degrees. I have a temp. gauge in the vent.
Has anyone else experienced similar problems with their AC?
The system can't be low on refrigerant as it just starts working properly out of the blue.
Thx for your help!
Anthony