Car being sluggish while in gear....?

I have a 1992 Chevy Lumina Euro 3.1L Sedan. The problem started last night. I drove about 40 miles, stopped and went to a mall for about 45 minutes. Came back to my car, started it (just fine), shifted into reverse and the car sounded like it wanted to die. Backed out of the parking spot, shifted into drive and it seemed like there was hardly any power or acceleration. It only does this while the car is in gear. So I drove it like this (had to get home), and while driving, I went to highway speeds (65-75mph which took a little longer than normal) and I was trying to listen to the car when all of a sudden it went back to normal operating ranges. At first, I thought the car was possibly slipping gears, but then I realized that the gears were shifting just fine. So I have come to the conclusion that it could be a sensor? If anyone has experience with this type of situation, or needs more info on what happened, just let me know and I will explain the best I can. I would like to get this fixed as quick as possible, but I know cars, and sometimes that is impossible. So, is it a sensor or something else in your opinion(s)?