gravity32,
See link for analysis of a Boeing 757 altitude data being used in serious incident investigation. The altitude data used in this investigation is FDR QNH corrected altitude data.
Are you sure you are looking at uncorrected data? Perhaps the math you are applying to correct data is causing a problem.
I claim no experience on Boeing aircraft but I am interested in accident/incident investigation.
ETA: Here is another link about pitot static system errors wrt to Boeing.
http://www.boeing.com/commercial/aer...23/aero_23.pdf
REPORT ON THE serious incident to icelandair BOEING 757