Just to amplify what others are saying, this is like saying you should cut Peyton Manning because he's had a bad start to 2008.
Elite DE's are IDP gold because they're rare; the best comparison is to elite TE's, who tend to be about as difficult to acquire.
Quit being impulsive and believing that you can control everything in fantasy football. Your starting lineup is always, except for those very rare and memorable games, going to have certain parts of it that perform well, and other parts that perform average, and still other parts that perform badly, and what parts fall into what category will vary from week to week. If you've constructed the roster well, you'll get through that all right because that's what happens to everybody.
If the worst problem you have on your roster at the moment is that Jared Allen is underperforming his draft position/market value, I'd say you're doing just fine.