There are many " undecided" voters who are looking for whatever excuse they can find to vote Trump. He will likely gain a few points only to lose them again next time he does something stupid.
If anything, candidates like Trump tend to underperform their polls. Look at Akin, Mourdock, and Wiener (in the mayoral race).
Yeah I have no doubt that trump will under perform in the actual results. The point is that Trump managed to somehow rebound twice, pre convention, and in mid September, and a final hurrah is very possible before the undecideds get serious.