I have the same issue (began with 7.0 and not solved yet). Also, if I show current frame, it will show a still on the Preview Monitor. When I press the space bar to continue playback, it flips the image up-side down.
Dave
MACPRO 2×2.26, OS 10.6.1, QTx and 7, two 24″ LED backlit Displays ATI Radeon HD4870 and GeForce GT120 cards.