Hi Arnljot,
These are complete & stick on solutions for the CPU.
LinkyI have an A4KD PSU which I ripped the guts out of. I have shoe horned an ATX 400W PSU in it & hard-wired the A4KD Mobo Feed into the ATX pcb. It's not yet quite finished & will need commisioning. It would be ideal for ample power to any heavily expanded Amiga. -but-
You mention Mediator so, guessing you have it in a Tower, that version of Mediator requires a P9 connector from an AT or ATX based PSU. If you are towered on Mediator, I recommend to a specific Convertor like this & you need to acquire a working ATX PSU obviously!:
LinkyRE the IDE, it could just be the knackered PSU casing the problems. Can you put an IDE CDROM on the header? - They draw a lot lot less power than a hard drive. - Could also be blown or leaky Caps on the Mobo. Have you looked for wet/sweaty tops? - Is the IDE cable sound?
Good luck there m8. :-)