Have you been getting blown out of the water every week or are getting beat slightly each week. I might stream kickers/ Defenses each week if you're losing slightly. They could make all the difference, especially the defense.
In my 18 team league, the 3rd highest scoring team is 3-7 because in 6 of his games, his opponents scored their highest point totals of the season. His average points margin in losses are the smallest in the league. No matter how well you draft and update your roster through the season, sometimes the fantasy football gods can be cruel.
You do have moveable pieces. If you're out of the playoffs at the point, maybe you need to move them and go for a home run trade. If standing pat isn't going to move you up in the standings, then you need to start dangling players to see if you can improve at QB (maybe a Brees or Rodgers) or at RB2.
Like the other poster says, you can stream your DEF or K, but that depends on the size of your league. If you're in a small league it works. If you're in a deep league, your mileage may vary simply because the free agent DEF's and K's are limited to table scraps.
Seeing that you have both P Manning and Stafford at QB you're pretty stocked, but closing in on the playoffs it might be good to try to fixate on one of them and maybe trade away the other one. Liking the rest of the team, but seeing as you have both AJ Green and Marshall at WR, and Harvin for that matter when he gets back, I'd see if I could pick up a high ranking RB like Doug Martin in a trade, maybe for Stafford, Bush and Harvin/Cobb. At TE Miller might not be as good of a choice with Big Ben out and Hernandez (hopefully) coming back. Might be an idea to put Miller on the trading block to see what comes in too. K and Def are better than average.
You'd still be set with Manning through the rest of the season and with Foster/Ridley you have 2 stable RBs and Charles having a bit of an up'n'down season while still remaining solid at WR with Green, Marshall and Harvin/Cobb.
The only players that I could say would give you a stronger team would be: QB Drew Brees, Aaron Rodgers RB Doug Martin, Marshawn Lynch, Trent Richardson, Frank Gore WR Vincent Jackson, Demaryius Thomas TE Jimmy Graham, Rob Gronkowski, Tony Gonzales K Lawrence Tynes (maybe not with the lack of action from Giants offense), Blair Walsh Def Chicago Bears, Houston Texans
These might be obvious choices and very hard to get, you might have to give away more than you'd like to get any of these but you're not playing your bench. Try to decide on who you trust and who you think you can afford to get rid off to get someone "elite".
First post, new to the forums, hope it gives some valuble input.