Edit window freezes during playback

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Has anyone ever found themselves in a situation where the edit window seems to be totally out of wack with jumpy or no timeline scrolling or when you try to highlight a waveform in a track the mouse click either responds late or not at all. Trying to edit a session with this happening is driving me insane. I know it's not the CPU or Pro Tools in general because other sessions work just fine. This is the second time this has happened and I can't figure out how to make it stop. I have tried saving the session as something else but the glitch follows me right into the renamed session. Please help. i have a 002 rack running pro tools LE 7.1 I have an RM 7100 spectral PC. OS is windows XP Home. another thing that happens is that the acivity in the edit window freezes like a crash. i know this to be true because the green gain bars will stop moving in the middle of the signal even though the audio is still playing back.