rpm's jumping when crusing
 

im not really sure how to describe it, but i have a 00 blazer 68,000miles, just had to replace the spider injector, runs better now but i noticed while i was crusing between 40-50mph at the same speed then started going up a little incline in the road that the rpms jumped up and down about 3 or 4 times jerking the truck until i mashed the gas harder.... not sure what this was about but it does it everynow and then?