You must have the thermistor plugged in when you are measuring it, as it should read around 100,000 Ohms when unplugged.
What model of printer do you have to begin with and what temps does it say it is reading just sitting there powered on.
Finally I changed to a new Rambo 1.3 board and tinkered with it the whole day yesterday. Now it works pretty well.
I suspect that I had run a large print without fan on and that may have damaged the board. Usually Ultimachine boards are manufactured high quality and tested with precision. I’m also a bit concerned because new board doesn’t have pins at X30- which I need to install ASAP before running large prints.