[Coco] Unreliable Coco3

[David Roper]

So, I got myself a new controller and drive combo (thanks, Nick!)

.. turns out that the main problem is actually my 512k Coco3 - the new
drive is showing precisely the same behaviour as the old one.

I've tried the same setup with my Coco2, and everything runs smooth and perfect

I suspect something is wrong with my Coco3's cartridge port, and would
appreciate any ideas

Symptoms:
When loading a cartridge (either through modified multipak or 'naked
in the port'), at least some of the time (1 or two times out of four)
the screen will be filled with garbage (green background, yellow
blocks) or, the entire display will be shifted a half-character to the
left

If it does load properly -
'DIR' with an unformated disk in the drive causes 5 'grinds' and an
?IO error (drive motor stops)
'DIR' with a formatted disk in the drive causes at most 1 'grind', and
then the drive motor spins endlessly
'DSKINI0' causes at most 1 'grind', and then the drive motor spins endlessly

The above is precisely the same symptoms as my old controller/drive,
and is 100% reproducible. Once again, the drive and controller work
flawlessly on my Coco2

Any ideas, suggestions, diagnostics? (or replacement Coco3s in Australia?)

Kind regards,
David