Transport bar blinking red and stuttery playback

[Noisy Chairs]

So I've been googling this problem and seen a lot of advice that it's a problem with the CPU being overloaded. This is only happening to me on one particular session (with less DSP in use than other sessions I've used as control for problem), so obviously there's something else going on. Anybody else have this problem with just one particular session? Is it something to do with a sample rate mismatch or something in that realm? I'm pretty stumped.

[cfillion]

You might find clues in the Performance Meter dialog. Right click in the top area and enable all measurements (especially the two RT ones and xrun counter). Sorting FX by CPU usage can help narrow it down too.

[Noisy Chairs]

Originally Posted by cfillion

You might find clues in the Performance Meter dialog. Right click in the top area and enable all measurements (especially the two RT ones and xrun counter). Sorting FX by CPU usage can help narrow it down too.

CPU usage looks just fine on all counts.

[Noisy Chairs]

I should add that this same session was fine yesterday. All I have done since then was to do work on another session, which worked fine. Opened this one up again, and it's all gone weird on me. Can open others and they work fine.