I'm thinking the weakest link in all this is the sensor, of course.
But even with a new cable, that is a suspicious item, too.
Tell us more about your statement of having a previous sensor that gives ok readings in the house but as you walk it out, it goes to 100%.
Are you saying that it goes from say 50 or so immediately to 100%? Or does it take awhile to go up?
Have you looked at the calibration numbers etched o the support bracket (if you have a pagoda) and checked in the 8000's set up to make sure the two calibration numbers match, and you aren't using the original 8000's sensor numbers with the old 25's sensor?
Any chance you have another piece of CAT-5 to test it with or could make one up pretty quick if you have the tool and a hundred feet or so of cable?
Just some thoughts. Dale