car jerks and stuff...?

my car (89 prelude si, 5spd) was running completely fine... I went my house, came back out like 10 minutes later, and started it... I started driving, and all of the sudden, whenever i let off the gas a little, it jerks really bad just once... like it looks a bunch of power.. it only does it when I lets off a little bit... then when it's parked it idles fine... but if I give it a little gas, it revs up, then drops down really fast, and continues to do so, until I let off the gas... I sprayed the vaccum hoses with WD-40, and it never reved up... so I guess it's not a vaccum leak? does anyone know what this is... I hope I made sence..