99 GT revs up and down

Hello! I'm a new member, although I visit this website frequently and appreciate how helpful everyone is. I have a 99 gt non-supercharged with 150,000 miles. I've replaced the plugs, wires, o2 sensors, maf sensor, and many more parts about 35,000 mi. ago. However, last week when I would enter the highway my rpms would pulse up and down and I could hear and feel the engine revving. It would drive fine after that at cruising speeds and around town, but would do this frequently when I would get on the highway and get up to speed. Then yesterday, in addition, I noticed that when I would accelerate as if to pass someone that my engine would sound a little harsh, almost like a grinding/buzzing/moaning noise, and I could also feel it on the gas pedal, its hardly noticeable but its there. Any suggestions as to what is the cause?

Not sure if what you are feeling is a misfire, vac leak, or possible TCC problem...no codes are set? Some codes don't turn the CE light on right away.....

No codes are coming up. As far as i know, the coils and fuel injectors haven't been changed. Think that could be the issue?