boot - Desktop PC won't turn off (turns back on)

I have an old (2006/2007) Medion 8818 desktop of which i replaced the broken FSP300-60GTM 300W PSU with a new LC500H-12 V2.2 500W PSU. It has no HDD. Thermaltake's Dr.Power II said my new PSU is fine. (The old one failed every test and was only good at keeping the standby LED lit.)

I turned on the PC, booting Trinity Rescue Kit 3.4 from DVD, and ran Memtest86+ v2.01 for 9.75 hours: 17 passes with 0 errors.

All well and good, aside from some corruption in the text background of Memtest86 that didn't change during the run:

And the fact that the machine bounced back on from power off and wouldn't boot past the annoying fullscreen Intel boot logo, like only the center logo of this question, no debug info and no USB devices connected.

I replaced the videocard with a Radeon HD 2400, and Memtest86+ now looks perfect. It still says the mobo RAM is fine. However, powering down the machine with a short press still makes it bounce back from the dead. Holding the power button for 8 seconds powers down the machine reliably.

The Phoenix AwardBIOS agrees that the power button should power down, and the clock is still accurate so the CMOS battery must be full.

On closer inspection, the front panel is connected to JUSB1 and JUSB2 connectors on the motherboard. I've removed those and turned on the PC... 1 Long Beep, 2 Short Beeps: Checksum error; dead mobo.

So it appears this question morphed into: Is the mobo the only faulty component here? (Besides the broken 300W PSU.) I don't see any damaged capacitors on it, but i can't see the ones under the heatsink (which i don't wish to remove as Memtest86+ worked fine):

Update: Turning the PSU back on, it gave the normal short fan spin. Pressing power booted normally. Trinity Rescue Kit 3.4 still hangs while loading initrd.trk, though. I've disabled Quick Boot and Instant On in the BIOS and also disabled Wakeup for USB and PS2 mouse. Unfortunately i could not find an option to show the boot diagnostics instead of the Intel logo. Pressing the power button during the TRK boot menu powers off the PC but leaves the PS2 keyboard's numlock on! No keys respond, though, not even Num Lock.

Answer

I've disconnected more cables from the front media panel, and one from the back media panel, and now DVDs and the HDD boot swimmingly!

I've also learned that Linux tends to take over the power button (hence only being able to power down with 5+ second holds), keyboard lights are always controlled by software, and the mobo powers the keyboard even when the PC is off.

Trinity Rescue Kit 3.4, Knoppicillin 6, and FreeDOS 1.0 detected no errors, but now i can't boot from USB due to isolinux: Disk error 01, AX=0201, drive 80.

I'll test the GeForce again as soon as i have a working OS.

Update: After setting the USB to HDD mode i was able to boot from it. Unfortunately, Zorin 7 appeared to need more than the 16 MB video RAM on the Radeon. To make sure, today i tried Zorin on the big 32GB USB stick... Removing the 32GB stick from the powered off PC i noticed it was warm. Subsequent boots yielded no USB response at all. As both sticks work in my new PC, i'm forced to conclude that the mobo is dying and should be harvested for parts.