Work through all of the steps (ideas) listed at http://ppro.wikia.com/wiki/Troubleshooting
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
I'm not sure this fits all the observed phenomena, but the first explanation that jumped out at me is that you have the Program monitor set to play back at less than Full resolution, with the Source monitor at Full which it just can't handle.
Thanks Smith - have looked through the doc you provided - but not too keen to reinstall CS5 quite yet.
This is a new computer with no other progs on it. I7/ SSD for OS (win7) and adobe progs/ media on a 7200 HD. Fully updated. Working fine till today. Only thing from that doc that jumped out at me is HDD space on my SSD - I am down to about 15% of the starting 60GB. I find it hard to believe that is the answer though.
To reiterate - the project runs fine otherwise... the system is capable of playing these files even without rendering in the main panel. It s ONLY in the source panel that they shudder - and only when selected from the timeline... Which makes no sense to me.
Jim Simon - good thought - i think that would easily explain the phenomenon - but was one of the first things i looked at. Even putting it at 1/4 wont let it play smoothly (but in the main panel i can watch at full res at normal frame rate!.)
Other troubleshoot I did was try a new project with the same source files... these have worked fine right off the bat. So it is something specific to the project that i am doing.
This is annoying as I have put a lot of work into it already - not keen on starting again!
If it's only this project, then something in it might simply have become smurfed.
Perhaps try importing the current proejct into a new one. See if that clears things up.
Tried to import into new project... same issue.
At least I can still edit in the timeline / make a new sequence for the rest of the project.
I would have liked to know the answer though