the past couple of days the truck has been actin up. Drivin up an on ramp and pressed down on the pedeal and started to speed up then started tothen the rpm started droppin then rasing it did it for atleast 5 miles straight before i pulled off the highway, it is worst when the engine is cold but will still do it when warm. after this started the truck started to smell like it was runnin rich. this mostly happens when im around 35 mph up 65 mph so a wide range, any ideas?

 

Lots of possibilities, maybe a vacuum leak.