F250 A/C, Very high pressures
 

Can anyone help w/my a/c in my 99 F-250?
The dang thing will cool fine and then the high side pressure will start to climb to 450 lbs. It will then shut off. After awhile it will start and it might cool for a long while or a very short while, even at highway speeds.
What makes the thing vary so much in pressure?
Fan clutch? Compressor?
It has 190,000 miles and is a 7.3.