A/C on/off with speed

Please help,
94 GMC K2500, 1/2 ton, 4x4,
Hear a strange whine that increases with accelleratioon for about six months. Then the other day my a/c stopped blowing cold when I slowed down. It worked again when I sped up on highway and did this warm cold thing for two days. Then it stopped blowing cold and I thought I smelled burning rubber. I'm going to charge the a/c with coolant and oil and I noticed I need new belt. If this doesn't work is the clutch next?

It's 100+ degrees here in August.

Do you have the gauges to check the operating pressure with the pump clutch engaged? It shouldn't be over 280 psi approximately during the pumping, on the high side. What you describe sounds like too much pressure in the system. Of course it could also be ANYTHING too.

Thanks Doug,
You are probably right, doesn't matter now though because my girlfriend didn't believe me that a/c was goofed until she ran it all day until she heard "a horrible screeching sound." Now compressor is obviously shot. Oh well