[panamajack]

Can you fill me in as to how you are utilizing the Pro Tools MIDI tracks? Are your tracks linked to another machine running VEP? Or are you using RTAS, like Structure, or other in-the-box VIs?

Also, are you using Sibelius as your primary score editor?

I am more familiar with the MIDI functions within Logic, but did play around with the full version of Structure running under PT8LE. I got it to play a movement of Mozart's Symphony 40 on a Nehalem quad 2.66, which was stable loaded with the included East-West orchestral samples. Never tried MIDI on such a large scale as yours on a PT Win7 machine, but I do have an HD2 PT8HD setup (also been playing around with the PT10HD demo) that works well for audio, for tracking and mixing.

I had to manually assign the correct clefs for the aforementioned Structure-driven Mozart file, because PT defaulted to all grand clefs, which made the score page unreadable.

Sounds like you are experiencing an issue with your graphics driver, if not inadequate CPU resources, or something running in the background that is sucking up your CPU cache.