Machine from hell

[16k_zx81]

Customer complained machine was running 'slowly' under load

Thought it might be hard disk (WD).

SMART - OK
WD short test - OK
WD Long test - OK

Plugged disk back into machine and POST identifies it as:

bzbzbzbzbzbzbzbzbzbzbzbzbzbzbzbz

Swapped out cable. Same result.

Test motherboard.

Eurosoft. Full test. - PASS.

Test RAM. MemtestX86 - 6 runs - PASS

Re-inserted drive, and got several 'flaky' results.

Put in a new WD drive. Recognised by bios accurately (WDxxxxxxxx), installed Win on it and everything OK.

Nuked and paved drive and it mounted (POST) OK. Recognised as WDxxxxxx.

Booted machine, got to windows, everything looks fine. Installed some software, updates, etc, and then ran PRIME95 (bearing in mind issue was 'under load')

Ran for 2 hours on PRIME95. No issues. Machine stable and running fine.

Got a phone call from customer this morning. Same weird hard disk recognition issue. Machine will not boot.

So:

If still problematic with second hard disk them must be motherboard?

Worth flashing BIOS on existing board to see if this makes any difference?

The only component in the machine that wasnt tested was PSU. Has anyone ever seen PSU cause this sort of issue in a HDD?

I hate this machine. Please shed some light!