mtt blues

I am pretty good at accumulating chips in the first 3-5 levels of a mtt. The problem is that I seem to always level out. Other players which have similar stack sizes during the 1st break always seem to pull ahead during the next few stages of the tournament. Say I have a stack size of about 50 bbs at the first break. how many hands should i be playing per orbit to stay ahead of the game from this point on?

Don't be playing under the presumption that you 'must' be playing a certain number of hands/orbit at certain stages of a tournament. Look for oppurtunities when they present themselves... play the table not just your cards.
Once antes come in, you do want to try to take down the blinds about once per orbit. Sometimes we're on tables where even that oppurtunity doesn't easily present itself (ie. lots of players opening before us and many re-raises preflop). If this starts happening to you then look for oppurtunities to re-steal instead. Shouldn't be hard to spot likely candidates for re-steal oppurtunities.
If mid to later stage levels of NLHE tournaments seem to be your sticking point, try picking up a copy of Harrington On Holdem Vol.2 Lots of valuable information in there.

I´ve had simular problems "jj white" where I dont seem to have any difficulty to build up a descent stack the first hour just to make all the wrong moves after the break.

The best advice I can give you is simply to tighten up when the blinds get bigger and only raise with hands that can handle a re-raise as these kind of plays show up much more frequently when the blinds get bigger and just as Poker Orfice mentioned if your gonna steal the re-rasie steals are much more effective as people have has a thing for beliving "the-guy-that-reraised-that-blindthief-with-AK-or-better"