Probably interference, could come from computer if you connect with USB. Try using shorter IMU cables, separated from other cables and ferrite rings if needed.
Yes I believe I get 12C errors, The battery is connected while connected while its connected to the computer. Is there a simple way to check or is it a re-wiring situation?
If you get I2C errors they should be visible in the GUI information window (bottom part of the screen)
Just verify IMU connectors and cables visually. If they are silicon cables pull them gently wire by wire, they could be broken inside. Do not pull hard.