Anybody notice this with FCP?

I have a 21.5" Imac and FCP runs so great on this thing. I started using a second montior for editing and when I use that second LCD monitor to view fullscreen video via desktop preview the playback is kind of jerky. I thought that maybe perhaps my system just could not handle running two monitors.

Now the interesting thing is if I flip the monitors around and use the Imac screen for desktop preview and the second monitor for the FCP panels and timeline I get full framerate playback. I can even manage full 720p 60p playback.

I'm thinking that perhaps my little Nvidia 9400 gpu is limiting performance to the main display which is why desktop preview works so much better on the main display then on the second LCD display. Does this sound correct or is this a known limitation of FCP? Maybe it is time to move up to a better gpu. To be honest just flipping around the displays really isn't the end of the word. My only concern is if I get a Matrox MXO which hooks up via the DVI port to output SDI if it will actually work or be limited in performance.

Thanks.

Imagine how much pixel space is being changed with full screen video on the second monitor. Now I don't know the specifics of how the second monitor out works on the iMac line but it makes sense that reversing the screens gives you smooth playback. The fact that the majority of the Final Cut desktop doesn't change much during playback, relieves the computational pressure on the second monitor out of the internal video card. Clearly the computer is built in a way that gives more power to the internal screen. Have you experimented with regular preview on the second monitor? Even on a MacPro you get better performance if you don't use full screen.

It doesn't matter what desktop preview mode I use. They are all equally jerky on the second monitor. I'm ok with flipping the monitors around if I have to but it does kind of suck not being able to use a Matrox MXO.

Would even getting a updated Imac with a better gpu even help in this situation or is a Mac Pro my only option? Now I know the Matrox MXO worked very well with older MBP's so I don't see why an Imac with a slightly better gpu wouldn't help.