strange symptoms in my truck.

[JMcQueen]

I have a 1999 C1500 with a 5.7L. I don't have error any codes. but when I am running my a/c and reach highway speed, and put it on cruise control, after a little bit it will drop out of it. Also, when I am going highway speed, my a/c fan doesn't turn very fast. But as I slow down it starts blowing hard again. I suspected that I have a vacuum leak. But I'm not having any O2 sensor lean codes. Finally, the last odd symptom is with my transmission, I think. it works perfectly. it goes through all the gears, then around 70mph suddenly it sounds like something in the drive-line drops out and the timber of the motor jumps up noticeably higher. but the tachometer does not show an increase in the engine's RPMs. I'm stumped and I need help.