I have the same issues with some of mine and its not the model itself, I have 5505s through 5540s. It is going to be a code issue in 8.2(2). TAC can say what they want but I lay my life on the fact its a bug.
What you might try to do is give 8.3 a shot. But watch out I think they upped the mem/flash requirements on it.
Outside of that keep plugging away with TAC. If your company has some pull you can ask for backbone TAC and get the guys in the US and they will get down to the "hey it is a bug" level much faster.
ASA netflow is a new feature so I am not at all suprised by its flakiness.
They also change the way NAT works in the 8.3