I've seen these symptoms a number of times before, and it's usually caused by a chip memory address fault. See my previous post on this subject where I'd hypothesised this. This latest clue adds plausibility.
From experience, it's most likely a fault in one of the DRAMs. Address faults are tricky to isolate to a specific device, so the best bet is probably to replace the chip memory DRAMs with new or known good devices. You'll need a good desoldering station as half of the DRAMs are soldered into the board in the A3000T.
And in case you've not done so yet, remove the socketed chip memory DRAMs and repeat the test as this will force DiagROM to use the bottom half of the memory. If the problem remains, then you've got the options of attempting some very complicated address fault finding, or replacing the DRAMs.
Would be useful if you can post the serial (text) output from DiagROM or post a screen shot, I don't do YouTube.