I would check the two 10uF caps that sit on the -5V line in the analog section and replace - they always seem to be bad. There are two further ones on the +5V that should be replaced too. I would personally just replace all the electrolytic caps to eliminate them as a source of the problem.
The -5V also goes to the serial port driver chip (M35) which could be bad causing a short/high current drain, particularly an output/input that was damaged perhaps by bad serial connections? >
