I would agree to a point that early in Peyton's career he didn't show very much promise in the post season, but I disagree completely in the notion that Peyton doesn't show up in the post season. Lets do a quick comparison between Peyton and Tom Brady's post season stats. Tom Brady has played in four additional post season games, so the stats will be a little scud, but I think the comparison is still relevant

Passing: 761 Attempts / 481 Completed
Completion Percentage: 63.2%
Passing Yards: 5,679
Touchdowns: 32
Interception: 21
Passer Rating: 88
Super Bowl Wins: 1

Passing: 887 Attempts / 553 Completed
Completion Percentage: 62.3%
Passing Yards: 5,949
Touchdowns: 42
Interception: 22
Passer Rating: 87
Super Bowl Wins: 3
*Photos and stats supplied by foxsports.com
Both quarterbacks have comparable numbers and again Tom Brady has four games on Peyton and Tom has two additional Super Bowl ring. There is also the matter of the team that surrounds the player. I would agree that Peyton had better offensive players in the early 2000's, but New England had a much superior defensive team.
Defensive minded teams will always win out against offensive minded teams in the playoffs. Why? With an offensive minded team, especially one that is pass first and run second the defense can isolate their efforts to the passing and not feel fearful of the other components of the opponents offense. In the later years that is what happened to the Colts, the running game was non-existent and the defense was able to focus on the wide receivers and shut them down. The team's philosophy is neither players fault, they can't choose what the team's philosophy will be, but it should also not be what determines their greatness.
As a Michigan fan, I am a Tom Brady fan because he carries the Michigan torch high and proud, but as a Peyton Manning fan the time has come to put to rest the notion that Peyton is not a great post season quarterback.