This is literally the *exact* same issue I've been experiencing - USB dropouts, that were initially mistaken as freezes (but weren't).
In the last few weeks, I've managed to not encounter one (knock on wood), but I'm not 100% certain they're gone, and I've changed so much, I'm not exactly sure what slowed them down/stopped them. Key things to note:
- Moved USB keyboard to one of the black ports on the back (HS09/HS10).
- Changed smbios to iMacPro1,1
- Update BIOS to revision g (or H) and double checked to ensure internal bluetooth/wifi card was completely disabled
I still encounter times in Ableton where the GUI and entire system (except for audio) lags for several seconds. I'm not sure why. It's quite annoying, but thankfully, the system hasn't seemed to drop USB out yet. I'm not convinced the problem is absolutely gone yet, but....this 3 things have seemed to improve it for me.