Ok, looked through the datasheets. One of the amazing features is "Low Cost!!"

Wonder why just two exclamation marks?

Anyway, found a few power supplies that should be suitable. They are rated at 9A nominal output current and have way more advanced features. The ones that originally came in your machine seem to have a feature for "Applicable to peak output wattage 200%" which sounds dodgy... There is also a limitation of 10s duration at peak current. I think that's why they are sort of suitable for the machine as all steppers do not run at 100% all the time and you just get power spikes when the arm moves around.
If I stick power supplies with nominal current way above required by the machine I think I should be on the safe side. Plus I'll be putting 3 PSUs instead of just two.
Having had so many power supply wiring problems with this very old model I just want to completely get rid of this issue and have the machine running at full speed reliably. I really like RV1S, just hoping that one day I can find an RV4S for an upgrade

Fingers crossed as I should be receiving a lab power supply today so should be able to check really quickly whether it's actually PSU problem. Will keep you updated.