A B717 Captain reported a deviation from a crossing altitude on approach to LGA partly as a result of a wake vortex encounter and partly an apparent FMC glitch.

Narrative

At 2;000 FT on the localizer 31 into LGA; outside of fix FABRY and once cleared for the approach the automation was configured for the approach - autopilot was flying. The FMA showed 'Prof to 600; idle to 1;700' (crossing altitude for FABRY is 1;700 FT). Approximately one to two miles short of FABRY the FMA changed to only 'Prof to 600' and the aircraft continued to descend; i.e. would have crossed FABRY below designated crossing altitude. I selected altitude hold and the FMA showed 'Hold 1;700'. As the aircraft was pitching to hold the altitude we encountered wake turbulence from the preceding aircraft; rolling us to thirty degrees of bank to the right; pitching the aircraft down and disconnecting the autopilot. During this wake turbulence encounter the aircraft continued to descend and crossed FABRY at 1;550 FT (i.e. 150 FT low). There was insufficient time to correct for the minor altitude discrepancy after I took over manually. I took control and re-established a stabilized approach and continued to land on Runway 31 in LGA. Suggest checking the programming for this approach - as the system and the FMA were depicting a standard localizer approach using PROF. The crossing altitude for FABRY was confirmed in the flight plan page prior to the approach and the FMA showed the correct information until approximately two miles short of the fix; as confirmed with the copilot. The automation would have taken the aircraft below the MDA for the fix (FABRY) and continued to the final MDA of 600 FT.