You need to provide step-by-step details, including the exact brand & model of all hardware items
A good way to start is to Work through all of the steps (ideas) listed at http://forums.adobe.com/thread/459220?tstart=0
If your problem isn't fixed after you follow all of the steps, report back with ALL OF THE DETAILS asked for in the FINALLY section, the questions at the end of the troubleshooting link... most especially the codec used and your project and output settings
Have you found an answer to this yet? I'm having the exact same problem. I record SD video using my Canon XH-A1 in 720x480, but when I capture in Adobe Premiere Pro on my new iMac (OS X 10.6.6), sometimes the clips capture in 720x480 and sometimes they capture in 655x480. There doesn't seem to be any rhyme or reason whatsoever to indicate why some do and some don't. This is extremely frustrating, as the 655x480 versions look terrible.
Unfortunately no answer or solution. UGH!