Help with fluctuating bed & hotend temps on Ender 3 with MKS Gen L board

So installed a MKS gen L board in my Ender 3, so I could set up a BL Touch without doing the boot-loader process (and because it's a better board, I hear). Followed instructions I found from Teaching Tech on YouTube. Got everything flashed and most things seem to be working, except the temps on the Ender 3 screen fluctuate wildly up and down by about 7-8 degrees on both the hot end and bed, constantly. If I try to print, it first tries to heat up the bed, which heats to the touch, but never registers a change in the fluctuation temp (it keeps fluctuating in the same range and doesn't go up). After a few seconds, it safeties out and gives me this message "Bed heating failed, PRINTER HALTED, Please reset". Another interesting thing I noticed is that if I turn off the power to the printer and power the board from the USB connection, the fluctuations stop and the temps are rock solid.

Thank you for any help you can provide.