Colt GTI - engine "hunting"

[Coltgti]

Hi i'm new to this forum so not sure i've posted this in the right place but please bear with me! My mom has got a 1993 Colt GTI which "hunts". This means that at some point during a journey, if we have to stop the engine revs to 2000 rpm constantly until we move off again. Also the car will keep going without any need to press the accelerator which is a little scary as you feel like you have no control (almost like driving herbie). The diagnostic scan codes were 21 - ECT sensor / circuit fault and 31 - Knock sensor malfunction. We have had the ECT sensor replaced. Adding a 900 resistor means the engine is unable to lower idle at 1300 rpm and the 350 resistor has not fixed the problem. Another odd thing is that whoever had the car before we did cut and re-welded all but the red wires on the air idle control bit to different coloured wires. We have had them re-welded back to the matching colour wires but don't know why someone would have done this and if it would make any difference to the revving. Any advice on what we could try next would be greatly appreciated as we would love to be able to fix the car