That's how adaptive mode simply works and well, if it is that severe and it degrades with even a simple comp, then your system simply doesn't have enough juice.
Most of the time when I scrub the timeline in my current animation it pixelates making corrections extremely difficult.
while scrubbing? Yes, as Myl says, that's how AE works. If you run a RAM preview and save it, though, you can scrub the RAM preview in the rez at which you created it. but anytime you make any edits, the RAM preview unrenders.
There are many ways to work with AE's various settings for view resolution. persistent RAM previews, test renders, and layer switches to maximize your editing experience. However, learning to use most of these interactive adjustments comes from many weeks (or, in my case, years) of experience with the application.
thanks everyone...much appreciated...Paddogirl