I had same problem after replacing arm & cable. Took it apart several times but could never sort it. I sent it away for repair, they sorted it saying was an issue with how the cable was fitted.
So double check how your cable is routed.
I had the same problem on my P3A. I too replaced arms & ribbon cable & couldn't stop it. I've now sent mine away for repair but will be interested to see what replies you get & how you get on. Good luck!