Problems with PC PSU (noise, ripple?)

[Tenacious]

Have you confirmed that earth ground and case ground are good?

When I get noise on anything I check this first.

Good point.  These PSUs are normally very well grounded to the motherboard ground.  OTOH, you want to be careful about running multiple GNDs to different places.  It could create noisy ground loops.  Maybe that is the place to start.

An Amiga will draw only a fraction of the 450 watts available.  I guess it's possible that it is not fully loading the PSU to the point of being stable.

Most scopes have very high impedance inputs, so as not to load the circuit under test.  I don't see why you could not directly monitor the 12VDC rail with it.

[Tenacious]

I bonded both GND and cable shield to ground within the PSU.  However, you're right - there's several other paths to earth from audio, video cables etc.  Perhaps I should have bonded the shield to earth rather than gnd/0v?

Edit: Just checked and there's 0 ohm difference between GND and earth in the PSU.

Hardware system designers are VERY carefully to choose only one point to tie all of the various grounds (analog, digital, shield, power GND) together, usually on the motherboard.  If you tie them together somewhere else as well, that is a ground loop.  

Your 0 ohm reading does NOT confirm that you have no ground loop problem.