snarkdluG

https://docs.dev-docs.org/htm/search.php?find=Sf314

Could you try with TOS v2.05 as well?

COPS or ZCONTROL is a substitute for XCONTROL. I'm not saying it's the solution, but you could try it if XCONTROL is crashing. It could of course be the MODEM accessory that is causing the problem. I don't know how the serial port is done or working on MiSTer. EDIT: I read that UART is supported by MiSTer so MODEM accessory shouldn't crash I guess.

Try COPS

EmuTOS exist as 192Kb and 256Kb. Some bits are missing in the smaller version. But excellent choice.

Mega ST can only handle 192kb ROMs (v1.02-v1.04). TOS v2.06 is 256kb so you need addictionally decoder hardware to get it to work.

The chip:

The chips creep out of the sockets after all these years. As DarkLord said, remove chips, use contact cleaner in socket. Reseat chip. So probably just bad connection. Cold solder joints or hairline cracks on the board could be hard to find if it comes to that point.

It's the Mega ST blitter patch. It usually fitted on a small board soldered directly to the 68k chip.

https://docs.sidecartridge.com/troubleshooting/#floppy-emulation

I made a comment about that in the thread when he made the space bar clips. It looks like you should be able to cut/file the the end of the space bar ones to make the return key clips. Nothing I have tried though.

You can download the files from https://www.thingiverse.com/thing:3887995. There is shops/sites that prints your 3D files for you. I have ordered from jlcpcb.com before and they look great. I'm sure there is some shops in UK that will do that as well.

Do you have a 35y old PSU in the STe? Kinda sounds like some capacitors is not up to specs anymore.

Exxos have them. But I'm sure Best Electronics probably have them as well.