One other physical pressure issue could be the oil relief valve. Its a long spring which should be a certain length and a certain tension, if its gone slack, it could be letting pressure past at the wrong point. Worth checking if you have the gauge rigged up.
I've got a sender on the bench at work at the moment, I was hoping to find something more reliable. I agree with above, this is probably the issue. It is a electro/mechanical device not very robustly made. I am amazed they last any more than a month or two!
