Colin,
Some of the TM500 plugins had notoriously unreliable IC sockets. I would certainly remove the socketed ICs, clean pins and see if anything improves. Several people have commented that the long term solution is to replace with decent IC sockets, I expect someone will know how to identify the unreliable originals. Have you checked for ripple on the +/-12V and 5V lines? Also it is worth exercising the range switch a few dozen times! Is the DM502 a new acquisition or one that was working OK but has now failed?
Regards,
Roger
