1997 Chevy Blazer Power Loss

I took my 97 S-10 Blazer in for a tune-up about a month ago. The filters and plugs were all changed out. A couple weeks after the tune-up I started noticing a considerable loss of performance. It starts up fine and idles fine. It accelerates and shifts fine until I get it up to about 35-40 mph and then it starts to struggle. The RPMs fluctuate rhythmically up and down as it tries to get above 40 mph, and this is especially so going uphill, no matter how slight the incline. On the highway, when I can finally get it up to highway speed, I can't pass anyone and can't hold speed going uphill. I try not to put the gas all the way down because it drives the RPMs way up. Yesterday as the day progressed it had trouble accelerating and shifting from a complete stop to 20 mph. The RPMs went all the way up to 3500 before it would shift into the next gear. My gas mileage has also dropped drastically from about 16 mpg in the city to about 12. I also recently had the O2 sensor changed.