I've never had my 6128 stable with the M4 and other cards, even though I used good quality high capacity power supplies. For the longest time I thought that my M4 was faulty... ...and this weekend I finally tried running the system with a 5.1v PSU, and all my problems vanished. I don't even need to power the M4 separately!
Obviously there's enough voltage drop across my CPC to make everything marginal when supplying it with just 5v.