So this problem started right after you updated the software?
Do you know what version of software the unit had in it before you updated it?
It sounds more to me like either the internal GPS Receiver could be going bad or that the electrical connection to the unit could be starting to fail. Low power to the unit can cause the GPS to fail as well. Check all electrical connections to the 798 unit and fix any that are loose or dirty. Be sure and check the fuse, fuse holder and battery connections too.