EEPROM cart and C4CPC causing instability with usifac2

[Cwiiis]

So for a while I blamed this on power supplies, but no power supply and no amount of cleaning and reflowing could make the usifac2 run stably on the Plus... But just today, I realised that it seems to be down to the cart. If I use the stock BASIC/Burnin' Rubber cart, the usifac2 works perfectly on my 6128 Plus, even if I use it via an MX4 expansion plane with another board plugged in. As soon as I use either the C4CPC or just an EEPROM cart (using this PCB with the NoAcid workaround: https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/amstrad-gx4000-plus-cartridge-pcb/msg174821/#msg174821), I get weird behaviour and reboots during usifac2 operation.

For a while, I did have some issues using the X-MEM and C4CPC concurrently, but that was resolved by using a better power supply. The usifac2 meanwhile remains unusable with anything except the stock cartridge. To be more specific about 'weird behaviour', I can always trigger this using the |help command - it starts printing out help, but after printing a few lines, it will show some corruption on the screen, then reboot shortly after - the usifac2 becomes non-operational until I power the machine off and on. Sometimes rather than corruption, it will act as if it was in tape mode and I hit ctrl+enter and wait for user input - if I hit esc at this point, the machine reboots and again it acts as if the usifac2 wasn't there until I power cycle.

We've been talking about this on discord and theorised that maybe the ROMDIS signal was marginal, but why would this be so highly dependent on the rom cartridge? Is there something about the acid workaround that's causing side-effects on the expansion bus? I don't really need the usifac2 to work necessarily in this configuration, but I'd certainly be interested in discovering why this is happening, especially if it may cause issues with other expansions down the line...

[genesis8]

X-mem cause problems with a CPC+ also FYI.

[Cwiiis]

X-mem cause problems with a CPC+ also FYI.

Works ok for me!