Yasas Randeepa

I am having an issue with my 406. the idle rpm gets stuck in 1500 range and just wobbles in that range after driving 50km straight. The worst part is if if i take my foot off from gas in gear the engine makes a sudden hard knock and kind of feels like the engine stops or something. So far I have replaced the stepper motor, did a carbon clearing of throttle and intake but nothing seems to work. I am going to replace the TPS also and see whether it will help. Has anyone had this issue before? can some one tell me or point me to where this issue might be coming from. Thanks in advance.