Sure, everyone is always oversampling Democrats according to you.
And a whole lot of other people. All those people saying that he can't really be up by that much? Oversampling is the most common explanation they give for why.
Actually, Fox News' polling team is suggesting that there's simply more self identifying Democrats these days based on multiple polling samples.
I'd need a cite for that, because it's the first I've heard it. What I've been hearing (from multiple sources) is that general polls showing Dem party identity (at least among voters) is lower this time around. One of the criticisms of polling that's been going on for months now (not just in the last week or so as your OP suggests) is that polling is sampling Democrats at a rate higher than they voted in the last election, and the conventional wisdom is that he had a high point of Dem identity and votes in 2008.
Now it's possible that conventional wisdom is all wrong, but it seems unlikely for Obama to have more support behind him and more people identifying as Democrat this time around given the demonstrably poor job he's done.
I'll point out that an additional angle here is differences between polls of likely voters and polls of the general public. Again, I've been pretty busy and haven't really been paying attention to this issue lately, but the tidbits I've picked up over the last month or so have all pointed in the direction of oversampling, not just of Democrats in general, but significantly with regard to likely Democrat voters.
And week to week polling aggregates haven't swung widely at all. I have no idea where you picked up that feel good theory from.
Obama wasn't polling 10 points up a few weeks ago. People then were saying he was polling high and pointing to oversampling as the cause.