Hi
No problem with polyphony then!
I am clutching at straws now, not much left.

But I do recall when I first bought my Novation impulse 61 key mid controller keyboard, have all sorts of problems of a similar random nature with midi data.
In the end this turned out to be an inferior quality mid cable (too thin) once changed for a thicker gauge all has work well ever since.
I just wonder if because, of the size and position of a large piano your lead may be a bit longer than the norm, mine is from the Clavinover and I made sure to use a thick one.
Below is a copy of an article I wrote explaining this on my web site .
It might just be the golden bullet.

Not all USB cables are the same.

Having spent many hours and sleepless nights in the past with USB connectivity problems. Like my Novation impulse 61 controller keyboard and my Yamaha Cvp 405 dropping their connection to my music software. At a critical point and loosing hours of work. Or the dreaded drive not available when using USB hard drives on my Cvp.
I thought I would pass this on. USB leads from the pound shop just wont do .
Since upgrading my USB leads to the spec below I have had no problems for the last 12 mths or so

Not all USB cables are the same. The thickness of the wires that supply power to the device can be critical to both audio performance,
and its resilience to static electricity. The best way to choose a cable is to look at the markings on the side. These will usually say '2725' and then another set of numbers containing 'AWG': '28AWG/1PR AND 24AWG/2C' or similar. The first number will always be 28AWG, and the second will be a number between 20 and 28. The lower this second number, the thicker the wire. If the printing just says '28AWG', this means 28AWG for both numbers. Focusrite recommends using cables where the second number is 24AWG or lower.

This is especially important if the device takes its power from the USB skt as my Impulse 61 does
As it will play havoc with any running auto keyboard mapping programs like Automap
If it drops its connection..

Also Yamaha keyboards don’t have much power to spare for running usb hard drives that are powered by usb.

Have fun

Rev 8-10-14

Good luck
Mike

BIAB2021 UltraPlus,AsusN55S1Tbssd, W10/64,Akai EIEpro
Yamaha CVP405,SquireStrat, CoolsoftVMidSynth
Novatation Impulse61 Ctr kbd, Cwalk blab Kontakt

http://mikesmusic.byethost16.com/