then you gave it too much slack. you need to have it so that it has just a little slack, but doesn't create slop in the gas peddle. if you have it too tight, the car will want to race or stay at a particular rpm. also, with the car off, take the throttle cable off of the throttle body and make sure the throttle body opens and closes by hand with no binding. if it binds when it is shut or almost shut, try tightening one of the mounting screws an 1/8th turn and see if that makes it better. I have found if you tighten the tb too much at one screw and not at another, it can warp the thing a touch and create a sticking point.
either way, tighten that thing so that when it is installed, you have only a slight amount of slack... say 1/8" or so that you can move by hand before the throttle wheel moves... post up when you do so. you can't just go completely loose one way and expect it to fix itself. you need to tinker a little, and it won't take more than a couple minutes...once you know what is up.