BSR will be restored when you exit the ISR. That chip auto saves and restores W,PC,Status, BSR, I think. So no need to worry about it from the Main point of view. I have not checked the banks you are using in the ISR, but I am thinking since it works sometimes they may be all in BANK 0. Could be the ADC is whats creating the NON bank 0 setting in the BSR.