[Rhinosaur]

As for when this first occurred...

I arrived at the studio (first day of a session with new clients!) and the Mac wouldn't boot. It was freezing on a pale blue screen after the grey loading screen. I was able to boot it in safe mode, so I tried a few suggestions I found online - mainly trashing preference files, but nothing helped.

Eventually I had to give in and rush it to the local Mac service centre, where they were able to determine that the graphics card had fried. They were good enough to lend me an identical graphics card from their 'spares' bin to get me through the session, while a replacement card was ordered.
This is where the aforementioned problem first appeared. Despite the graphics chugging slowly I was able to get through the session without any other Pro Tools issues.

Recently the brand new card arrived and was installed (again identical to the original), but the problem persists.

I have ran TechTool which came with the APP, and all hardware passed the scan without issue (including the replacement card).

Basically I have no idea if the problem is OSX or Pro Tools related (perhaps due to my initial attempts to get the system to boot), hardware related (maybe something else failed when the gfx card did), or just a combination of the gear I'm using (which I refuse to believe as it's been working fine for 3 years until now).

FWIW the only other recent change: I upgraded/crossgraded to a PT9 license which I've been using on a laptop when not at the studio, but the studio computer has remained on PT 8 HD and 10.5.8 (see previous post), which the PT9 license on the iLok seems to allow for.