Paul

Ive certainly had your configuration up an running with no apparent problems on a gen1 intel MacPro with X1900XT and TH2G at 3072x768 @75Hz on OS10.4.11 with QT7.3.1 - running m165 (and m167)

I dont know what customisations you may have in m171 - However, a quick test would be to use the current release version m165 to see if you are still getting the same issues...

There were also some murmurings about quicktime versions recently - Im still running QT7.3.1 - Apple did something in QT7.4, not sure if its fixed in recent QT version - check with Richard.

S