overload problems - PT6

I have similar issues with similar hardware, but not as extreme as you're describing, when using Pianoteq as a plugin.
When using the standalone application the performance is much higher.
Also you can try to increase the buffer size, this will help to get over small performance spikes without cracks.

My suspicions lie with the Motu (esp. if it's10yr old plus).

Could you try swapping out the Motu? (borrow or test from a friend or in a store)

I don't know if I'm right about this hunch.. but if it's an older one with no physical knobs (with CueMix software for volume control, and effects) looking at volume and other settings within that software might be my first thing to eliminate from the equation.

Thought of that because it maybe doesn't have simple metal pots to get dusty/rusty. But that's a physical possibility (an external unit I have, has knobs which have been the worst case of this in my lifetime - pots going south in month 1 is not my fav thing - BTW, that co states that users must be gentle with knobs or damage to pots can occurr.. riiiight guys.. more like 'our pots are known to make your audio jump from clean to scratchy - and we like to blame you, our simp customers who we just know are hammering our poor hard working knobs with an oversized truck spanner when we aren't looking").

That CueMix can instigate a series of internal Motu unit effects, within the DAC loop which was a nice feature.. but I'd also make sure to disable any of that, like a hard compression (which might be kicking in sending you into saturation and bleed seemingly randomly because of some threshold).

In general make sure no level or other FX are set too high (gain staging).

With lowered output from Pianoteq, and adjustment of any input and output volumes, you can alter the harshness/pureness ratio of your audio signal (to ear, and to record).

If the Motu runs native 44.1kHz and 48kHz sample rates, I'd really want to use those instead of 32kHz which is probably a quick and dirty DAC routine with a lossy and smoothed over nature (making sure from Pianoteq, the ASIO drivers, the DAW, the Motu software etc.. are all set the same rate).

Disable any internal OS or sound card audio drivers, making sure only to enable your external unit drivers (in OS audio panel).

That's a start as well as what others well mention above.. but lots of people here have given all sorts of advice about all kinds of possibilities in various systems, maybe worth searching the forum here to view some of those, if you're still really flummoxed and certain it's not something typical.

For way more about ordinary system fixes for poor audio performance, I really like this guide from Cantabile software..

Glitch Free - in-depth guide to tuning Windows for reliable real-time audio performance

(assuming Windows - just because this kind of thing seems more likely in a Win sys from my exp.)..

Hope some of that helps out.

I would get a MIDI file with a lot of intense/complex passages, and play it to test audio output options.  I get the best results with my soundcard's ASIO driver, but test the different output options and configurations.  If you're performance index is that high, it's almost certainly not the CPU and either a settings or soundcard issue.

If you use the ASIO driver, set Pianoteq to something like 2112 samples or more and if there's still an issue with live playback, that basically guarantees that it's not the computer/cpu but something else in the signal chain or configuration.

If you don't have an ASIO driver, you can fall back on using something like ASIO4All, but the native ASIO driver--even on older interfaces--will usually be the best option probably better.  My soundcard is old too, and it requires the drivers to be installed in a very complicated process because they stopped making drivers for either XP or Vista, and so to get it to work on Windows 7-10, I have to install a third party driver bundled with another software pack that doesn't work that's older than what the manufacturer has on their site, and then install the driver update from the manufacturer or it doesn't work.  So, you're absolutely right that old drivers can be quite temperamental.