Depending what your AIS spits out you may need to tell the Lowrance to ignore the 0183 positional string.
Not AIS, but- I reckoned I would use my handheld garmin as a trip computer (less clutter on the plotter screen) and also as a backup GPS antenna. (I've ended up with an NMEA patch bay in the console, but that's another story). What I found was if you feed it the location from the 2K network (antenna puck) and then superimpose the handheld's position (inevitably slightly different) it goes nuts! (and creates a very fuzzy trail)
I assume the HDS uses similar logic / menus to the non HDS stuff - there is a check box in the NMEA0183 setup screen that says something to the effect of "use me for position" - might be worth looking at if it starts to go bonkers on screen!
All of course irrelevant if your AIS box doesn't tell the plotter where it thinks it is! (e.g the new DSC radios with built - in GPS)