Check that the isolated +5v supply is clean and at +5v.
Then check that the outputs from the clock and data opto isolators are switching between proper logic levels.
Sick clock or data bits going into the serial DAC will drive the output nuts, as might noise spikes on the 5v supply.
My money is either on a sick opto, or a faulty decoupling cap.
The battery may have changed the supply voltage just enough to get a marginal opto functioning.
It may not be noise on the supply, but a dying opto isolator.