How do you know the temperature varies that much? Are you printing on glass? Is the glass making good contact with the entire heat bed? If it is not, the heat bed will be warmer in those locations of poor contact, while the glass will be cooler.

My guess is you are not fighting poor conductivity. But you can scrape off some of the red silk screen in different locations and verify each rail (and line) is getting the same voltage. I would do that before soldering big wires on to give more even current flow.