First of all, two interesting articles for this matter.
One about clutch players, understood as the ability to rise his game level when facing a break point, which shows that Sampras played less well when facing break point than when facing other point: http://www.tacticaltennisblog.com/clutch-players/
It doesn't tell however, if he served less well.
The other is a video about the slowing of Wimbledon, and also the higher bounce: http://www.youtube.com/watch?feature...=Gjxf8RjhKXI#!
The increasing of service hold might have to do with the homogenization of surfaces, which allow the better players to dominate on every surfaces because they play, in comparison as how they used to play, roughly the same.
It might as well have to do with the slowing of the surfaces, which give an advantage to the better baselined. A lesser server can still defeat a big server because he can hold is own serve, not with a big serve, but with a better ground game.
None of these reasons could fit for Roddick though, as he isn't one of the best baseliner for a long time now.