The IFD550 is capable of showing Vertical and Lateral Deviation on the SVS page.

If the IFD550 is paired with another IFD5XX/4XX Navigator the vertical and lateral deviation information could present a flashing behavior if;

The IFD Navigators are not both at the same NAV Database cycle.

This could consequently show deviation information on an interfaced Indicator. 

The issue is that the IFD when connected with another will populate the vertical and lateral deviation information and cross reference with the No.2 IFD. If the Nav cycle is not the same it will disable the deviation and attempt to display it. Once it shows, it will cross reference again and remove it. This will continue to occur until the NAV Databases are corrected.