may be a bit of a dumb question but if the tyres r seconhand they may be out of round, have a flat spot somewhere on them. if not then common thing is brake shudder especially if youve just been getting in the car and using the stoppers from cold then that would definately be your problem. otherwise if all else weve mentioned has been done and u r 100 percent that alignment and balance are spot on then only leaves bushes or tie rod ends or involves u taking a look at the front suspension to find out. who knows maybe the guys who installed the springs left a bolt out or lose , ive seen it b4 wouldnt surprise me.