It's not any of the caps, as a shorted one would burst, or tax the power supply section enough to blow the AC input fuse. Not the power cord because the other chnl works fine, which is connected internally to the same AC input connection. If you have conventional speaker cables, and the speaker does work without intermitent static from loose connections, then it's not at fault either.
Nothing during playback indicated any differences in speaker sound, giving suspect to network or driver shorts?
When you swapped speaker cabling, the now "left" speaker chnl quit as well?
Have you crossed (not swapped) the interconnect cables into the Portal, thus identifying a suspect CD player chnl.? Try this as well, with all the original cabling configuration.
If applicable within your amp, there might be a protection relay being switched for several reasons. One application would be from a faulty or incorrect thermal sensor located internally on right heatsink. Next time the Rt chnl cuts out, rush over to the chassis and listen/feel for mechanical relay actuation. Again, this won't reveal the problem, but confirms issues with the amp.
Keep us posted...