1987 Honda Civic Hatchback jerking back and forth

I have an 87 Honda Civic hatchback that has developed a problem after about 10 minutes of driving. The car feels like its jerking back and forth when pressure is applied to the gas pedal. Its a 4 speed and otherwise in excellent condition, has a clutch thats only 3 years old. When in 4th gear and going about 50mph, the car drives a bit smoother, and when I take my foot off the gas, the problem goes away altogether, but going slower than that in either 2nd or 3rd gears, the vehicle feels like its trying to push itself forward. Going up hills is nearly impossible in this state as well. I only paid a few hundred for the car and am trying to make it last just untill I get through school. Hopefully without paying more to repair it than I paid for it to begin with. Ideas?