Bob, check if OpenGL acceleration is turned on. If not, try turning it on, if that doesn't help leave it on but turn off all advanced features. If OpenGL acceleration is already on try turning it off. Of course make sure you have the latest graphics drivers. Check if your video card is supported, there's a list somewhere on the adobe site.
Overall I like CS4. CS4 Bridge is far far better than CS3 bridge, and i'm slowly getting used to Photoshop CS4. Not a fan of the adjustment panel, it steals space. Also the fit image script is broken, it doesn't work properly within actions.
MC1, to use 5DII files inside CS3 get the free adobe dng converter, convert your raw files to dng, then open them in CS3. You don't lose information, as I understand it it's still RAW and in the original format, it just adds metadata describing the raw format. Or use DPP.