[JFreak]

Hi, welcome to the community.

There are two easy self help repairs to try.

First one: buy a new firewire cable. It sounds to me that it is a clocking problem, and as it also seems to happen with internal clock (without digital devices connected, try it if you alredy didn't) then most likey culprit is faulty firewire cable.

Second one: open it and reseat the power harness. You will se a Y-shaped multi-wire power connector that has about inch wide plastic ends. Take it out, clean the contacts with industrial alcohol, juggle a little and put it back just like it was. Then try again.