Besides re-soldering the opto connections, which i think you've done? ....

Wasn't there a problem like this (was it MartyJ ???) where an ongoing opto problem came down to being a faulty opto??
Once replaced, fault was cleared!

Could this be the case in this machine?

There was mention of either faulty opto's that dont last long (although they should) or bad design running too much current thru the main opto-led, killing it over time....

MM.