samsaridinc1500

So I have a '96 c1500 5.7 2wd A/T. A little while back I was driving down the interstate at about 80 mph, which the tach said was about 2400 rpm. Then it suddenly revved itself way up to about 2900 rpm and the truck actually slowed down. I could feel it, like engine breaking on a motorcycle. It didn't sound to me like it completely shifted down, though... I don't know. I realized that if it happened I could let off the gas, let it idle down to about 23-2400 again and get back on it it'd be fine for a minute. Eventually I noticed that if I ran about 70-75 mph (roughly 2200 rpm) it was a little more stable.
Now it's started to do the same kind of thing at lower speeds, sometimes just around town. It also started to hesitate sometimes when I'm speeding up from a stop.
I just replaced the fuel pump, fuel filter, air filter, tps and starter. The tranny was rebuilt just a little while back.
Any ideas welcomed. Thanks