Severe performance problem during project preview play in Fedora 16

I'm running FC16 on a quad-core box with an Nvidia GF8200. I upgraded my machine from a dual-core running FC14 and an older Nvidia card.

Kdenlive worked OK on the old box - a bit slow, but usable during the play-through of the project. Now I can barely play any transitions - it's not usable any more.

I have blacklisted 'nouveau' and am using the drivers suggested in Fedora Forum's Nvidia thread. The Kdenlive version is the standard 0.8x binary which is in the FC16 supporting repos.

Any thoughts? I've messed around with using various graphics (OpenGL, X11, etc etc), restarting Kdenlive and nothing seems to help so far.