I think the playoff loss to the Steelers ruined him. No matter how well he "manages" a game during the season, no matter how much confidence he builds up against lesser opponents, and regardless of home-field advantage and a season's worth of accomplishments...the pressure of the playoffs is enormous. Plummer just isn't that kind of QB. And I think that's been confirmed in his own mind.
He can't play the playoffs again this week. He has to go through the entire season again. It's easy to lose focus, because he knows that nothing can prepare him to return to that pressure, and nothing he can accomplish will make the looming post-season any easier. I think he's still "losing" to the Steelers and maybe it's taking time to build up his game-day mentality to an acceptable level.
That's my .02. I could be wrong, but he seems to be regressing. Losing to the Colts when they're the better team is one thing. Losing to Pittsburgh when you were (supposedly) the better team is painful.

at the Dime Store Psychology. Is that your professional opinion, doctor, or are you off the clock right now?
His big throw to Javon was both off-target and underthrown. It just so happened that he was in single coverage and the DB was burned badly so was able to adjust. Now that Bell has the starting job locked up he will put up good numbers as the Denver D will always keep them in games. I don't think he will put up great numbers until Plummer gets out of his funk.
I didn't think he would take the starting position outright though so my Denver O predictions aren't off to a good start
It also came while he was rolling left and traveled 54 yards through the air. 95% of the QBs in the NFL wouldn't have even been able to make that in the first place. Geez, what do you guys want from him? Pinpoint accuracy all over the field, including 50+ yards out, whether inside or outside of the pocket? This isn't Robo-QB here.
There was a 4-game stretch in 2004 where he threw 2 TDs to 9 INTs and everyone started speculating about what the heck was wrong with him. He responded by setting a Denver record for consecutive passes without an INT last year. Jake Plummer is still Jake Plummer. If it weren't for severe drops in week 1, he would have had a pretty respectable game. One of those interceptions that he tossed went right through the hands of his target and into the hands of the DB waiting behind. Another one was a desperation INT. He also had two sure TDs dropped, and played the game with minimal time to throw. If Plummer had gone 16/26 for 200 yards and 2 TDs to 1 INT (which he would have been had those three passes been caught), would we be having this conversation right now? That would give him two 200+ yard, 2 TD games in five outings. It would also give him a 5:3 TD:Int ratio on the season. And it's not like Denver's played the same schedule as San Diego here... even with the Oakland game, Denver's opponents have combined for a winning cumulative record so far (14-10, iirc). Prior to this week, Denver had faced the toughest schedule in the entire NFL according to adjusted opponent winning percentage (and the lead was so huge that they might still have it), and Plummer has faced Baltimore, New England, Kansas, and St. Louis- some solid defenses there.
It's not like Plummer has forgotten how to play, here. He's had some bad breaks, faced some tough defenses, and had Shanahan call the most conservative games he's ever called in his history at Denver. I'm still thoroughly convinced that he's still the same quarterback that he was last year.