Hello, just a few possible things:
* Silly question you are using the reset key on that fault right? as that will clear the diverter fault...
* You could probaly check the obvious things like making sure all boards are firmly seated.
* MK5 boards are generally over 25+ years old now, the error isn't the coin mech itself (usually) but areas of the board handling i/o etc. But also concerning that it didn't remember its settings on restart.
* A few things, the larger capacitors need to be checked and or replaced, I would replace the battery (can easily get online or from Jarcar). Seem to be a classic as the board warms up you have issues.. You could try letting it warm up for say ~30 mins, let it error out, restart, re-setup everything and then it "might" run fine from then onwards for the next few hours (obviously that's not really a solution). - looks like you do this already and when you put a coin in it tips it over...
* You could run the game without the coin mech at all and use notes if possible? but again, the fact on restart that it's lost it's NVRAM isn't good.
* The best thing is to get another main board.
* It could also be the PSU as if its giving dirty power these boards are tough enough but can only take so much. (same thing, as it warms up it's not working properly).
* Once you get a spare board if that also has issues similar then yeah the PSU is next to swap. (someone here might help with that).
* There are people who can repair boards, clean up (usually dusty old cigarette smoke) , check for dry joints and re-solder bits that are obvious.
Hope that helps!