I am experiencing weak flippers on my Williams Fish Tales #FT. After a mechanical revamp of the flippers (exchanges bushes etc., but that did not help) I discovered that the flipper buttons are intermittent (discovered by using Test switches and Test edges in the Test menu) with a high frequency (50 Hz or so). So I did the following:
1. I cleaned the optoboards at each flipper button, and even exchanged them with another pinball machine, but that is not the cause.
2. I replaced the fliptronics II board with another one, problem is still not solved...

The flipper EOS switches are also connected to the Fliptronics II board, but they are not intermittent, only the flipper buttons (2 switches per button). If it would be a power problem, the EOS switches would also be intermittent?

Now I don't know what to do next...I would appreciate any suggestions.

Your comment about the different principle between opto's and mechanical EOS switches made me look closer to the power grid. Eventually I found the cause in the 12V circuit on the power board that powers the opto's. The condensator C30 was loose and did not smoothen the power enough after the bridge rectifiers. Resoldering C30 did the job! Flippers are strong again.

Problem solved.