Hi 1Lav78,
Welcome to the forum.
For any heat modification to a Bradley, especially the Original and New Original models (non-digital) I recommend using a PID or some other style of heat controller.
It is difficult to help you when you don't post a reference to the instructions you used for your modification, but in this case it does look like a PID alone would not resolve your problem. If you can provide a link to that that would be helpful.
I've never experienced an OHM measurement drop during a reading. Check the batteries in your meter, but I would suspect there is something wrong with your 900w element.