Blanks steadily increasing

Our FP628 Nitrogen Analyzer passed System and Leak checks but %Nitrogen steadily increases for successive blanks. Re-running system and leak tests gave error message: Combustion & ballast pressure failed. Immediately re-running tests came back normal.

Next day, another leak check was performed for whole He which indicated a "massive nitrogen leak" (O2 was normal) yet a retest immediately after "passed". What is going on? Any help would be appreciated.