You should not edit by looking at only 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 “flicker” is absolutely normal and is due to video “frames” being made up of TWO “fields”.
If there is a lot of action 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.