SM-P900 here. Bought in Best Buy at the end of July. I too have the flickering screen sometimes. Totally stock.
I don't know what to do about it so I have just been ignoring it. Never even asked about it in the store.
There are some lag issues that I don't think a brand new very expensive device should have either but I figured the lag was just because of touch wiz so what can I do about that if I don't want to root? (Nothing.)