Sounds like you're experiencing judder. Read more about it here. It's not specific to post work either; you can get that same problem by panning the camera at the wrong speed when filming. It's an issue Hollywood has been dealing with since the advent of cinema.
Yeah, had a read of that earlier, thinking it could be that issue, but, I have projects that where nice and smooth, and now have 'the judder'. So I figured it can't be that... something has changed somewhere. It also appears to be effecting other ellements such as objects fading in and out using opacity. They're not appearing 'smoothly', they're crashing in for want of a better description...
Have you updated anything recently? Your graphics card, your OS, your AE?
Speaking of AE, exactly what is your version? 11.0.4? 12.2.0?
Nah, nothing updated or changed.
Try updating your AE. You're missing the 10.5.1 update. I'm not very optimistic that it'll solve your problem, but there's a chance.
Obviously something has changed. Maybe some of your hardware is failing (you do have backups, right?), maybe you just need to trash your AE prefs, or maybe now is a good time to consider upgrading to the newer version. There are a lot of nice, new features and it's a pretty cheap cost to upgrade from CS5.5.
A change in frame rate will change judder. A change in the number of pixels your objects move will change judder. A change in line thickness will change things. Going from interlaced to progressive in your render will change things...
If you can step through your problem area a frame at a time with the Composition Zoom Factor set to something 400% and check the edges of your details and see any kind of juddering edges then it's an anti aliasing issue caused by thin lines and position inaccuracy.
If your edges are nice and smooth at 400% and a frame at a time and a Delivery Format movie is having juddering problems then it's a combination of frame rate, in the case of a computer, refresh rate, and motion in the frame. You'll have to change frame rate, or speed, or both to make the problem go away or mask the problem with motion blur.
I was thinking it was judder until he mentioned that his opacity changes weren't smooth AND that previous projects that were fine are now stuttering. Ash, just to be sure, can you create a new comp and animate the opacity of something? Is that smooth?
Thanks for all the input guys.
I've spent that much time staring at the screen and motion I think I might be going a slightly mad.
Having done a few more tests on older projects I can see it more in some comps than others, but I'm not sure if I'm not tricking myself and thinking it's worse than it actually is by being overly critical. But it's definately not right...
As it happens, it's being upgraded to the latest AE in a few weeks time, so I can use 60fps work around until then if I have to.
I'm off to have my eyes and head tested.
Again, thanks for the input guys, it's much appreciated.
I've also seen this sort of thing when using the wrong framerate on video card. If my comp is 30 and my video preview is set to 23.98, things look really crappy. Maybe someone got into you machine and changed the video preview framerate???