Putting my "eliminate possibilities" hat on, do you have / can you borrow a handheld that squirts out NMEA0183?
I have mine set up such that if I loose the main antenna through some random accident with the A- frame / cabling, I can dive into the console, do a quick re- patch & feed a lat / long into the plotter from my trusty Garmin 12.
Ok, you may need to muck about with choc blocks etc, but if you can feed it a signal from the 0183 in and it does the same thing than you know it's the plotter and not the antenna / cabling. (remember to disconnect the 2K antenna first or you may see some strange things on screen - unless the HDS can average thre two?
Other thought - does this happen regardless of what you are doing? e.g sat in the marina for 5 mins vs <switch on - blast out & be doing 76 knots> or you always have to cruise down a river to get out & it always happens by the pub on the third bend every time kind of thing?
That might eliminate some form of signal jamming - whether it be a radar or an evil nerd. GPS signals are at best weak on a good day.....