You should not attempt a serious edit by only looking at the computer monitor for judging quality.
FCP is designed as an editing SYSTEM to be operated with an external video monitor connected
(through a camcorder is just fine) DURING the edit process.
The external video monitor would have shown you that many video “freeze frames” will wiggle, dance, jitter, flicker, etc.
This Still-frame “flicker” is absolutely normal and is due to the fact that video “frames” are actually made up of TWO “fields”.
If there is a lot of ACTION (or motion) in the frame you chose to “freeze”, the 2 fields will appear slightly different from one another and will jump back-and-forth between themselves, over and over during the “freeze”.
To solve the problem, just add the “Flicker Filter” or, better still, the “De-Interlace Filter” to the freeze(s).
Effects (tab) >
Video Filters >
Video >
De-Interlace (or)
Flicker Filter
Again, if you watch the external monitor, you will see the flicker go away after a filter is applied.