Hmmm... Honestly, my first response is the same as petew's -- you are dealing with high voltages and currents and could easily hurt or kill (yes, kill) yourself or the amp. I would never hire a tech who couldn't use, or was unfamiliar with, a voltmeter!
If you are determined, start by turning the amp off and unplugging it from the wall. Leave it for 10 - 60 minutes, then measure the d.c. voltage across the caps. Make sure it is zero, or durn close.
Now, trace the wires from the a.c. line input to the transformer. One side probably goes pretty straight to it; the other likely goes through a fuse and switch before the wire goes back to the board and then to the transformer. that's the primary side, and will lead to two wires into one coil of the transformer.
The output, or secondary, side may have several groups of wires. These generate different voltages, for example a low-voltage circuit for any logic circuits and the front end, maybe a medium-voltage one for the driver stages, and a high-voltage coil for the final power amp. Chances are you'll be able to tell by the way the wires are grouped on the PCB (printed circuit board). Check the voltage and make sure it is zero across all the wires (measure from each wire to ground and then across the pairs). Then use your ohmmeter and verify that the resistance between the pairs of wires is low for each circuit. (Using an ohmmeter on a charged circuit can fry the meter.) Gently prod the wires and make sure continuity is maintained (no loose/broken transformer wires). this won't check the wires deep in the transformer, but may indicate a bad solder joint or wire broken right as it comes off the transformer and/or into the board (common stress points).
While you have it apart, take a bright white light and shine on the back of the board. Look carefully as you gently push on various components/wires and see if there are any hairline cracks in the solder joints. A bad joint can be hard to detect, and if it's intermittent may be almost impossible to see until the circuit warms up (maybe not even then).
You can try to check the rectifiers (diodes) with your ohmmeter -- should read high one way, low the other. Other circuit elements may make it impossible to tell without removing them, however.
If you are comfortable playing around with live high-voltage circuits you can power up the amp, clip the a.c. voltmeter to the various transformer outputs, and see if anything happens when the problem occurs. You can also check the d.c. output voltage across the various filter capacitors, again watching to see if the meter fluctuates when your noise happens.
Honestly, you are playing with fire, and blind probing is very hit and miss for something like this. If you aren't terribly familiar with using a voltmeter, I would do the power-off tests, then if nothing obvious shows up, take it to a tech with the equipment and experience to troubleshoot it. You will money, frustration, and possibly a lifetime ahead. The voltages in a power amp are way above any computer/game rig (past the incoming line).
FWIWFM - Don
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley