MIDI timing issue

[Reynard Foxe]

Hi all,
I'm in need of some help. I have started using ProTools 10.3 on an HP i7 quad desktop. For MIDI, I have a bunch of external sound boxes connected via a MOTU MicroLite (5 in, 5 out).
Here's the problem; when using MIDI, it records well and quantizes fine but on replay the timing noticeably moves around. It sounds similar to an unquantized track. And it is random. Every time I play it back, different notes will be out of time.
The problem was still present when I skipped the MOTU and used the MIDI output on my Presonus AudioBox, so I don't think the problem is the MOTU MicroLite.
All drivers are up to date. No other programs are running. The only USB things plugged in are the keyboard and mouse.
It happens on the USB2 and USB3 busses.
Even when I mute all other tracks the weird timing things still persist (eg playing just the Hi Hat track against the click).
I have done all the optimizing things I can find on the Avid site.
Any ideas? I would be so grateful.