
Now on to investigate WHY..
The jamma harness I'm using is from jammaboards.com, I purchased 3 of them Jan 16 2008, haven't had a use for them until now.
This harness uses a single thick wire for each current, i.e 4 cables for 5v,12v,-5V and GND, however on closer inspection - they are 22 AWG, same wire gauge used for buttons!?!?!

22 AWG is totally unacceptable for power cabling, even at a low 25W (5A@5V) power usage and 3 feet length of wire, you get a 0.5V voltage drop, increase the length to 6ft, or 2meters and you get a whole volt less at your edge than at your PSU. UNACCEPTABLE!
Now, jammaboards.com state that they use 18AWG for power, I didn't check this when I got them 3 years ago, so hopefully they have changed - buyer be aware..
Anyone know where to get a quality harness for a reasonable price?