my civic will rock back and forth very violently in 1st gear at around 1.5k rpms, but if you give it more gas and get it to around 2k rpms or more it wont jerk anymore and everything smooths out.

Im thinking its either a tune issue or a timing issue (maybe timing belt is off by 1 tooth?)
Ill check the timing tomorrow but maybe its also because of an upgraded clutch (I bought the car secondhand and im not sure if it has an upgraded clutch)

 

Loose subframe bolts, or broken mounts.