Lowrance HD 5

I posted earlier this year about the HD5 being slow in the GPS mode . The boat on the screen is lagging behind the actual boat on the water. I have loaded the 3.5 software as reccomended by lowrance and folks on this board. It may have helped a little but there is still a delay.. and when you are trying to fish a small brush top it makes it a nightmare to get on the mark. Ive been advised to add an external antena .. disable or enable WASS.. will that help? I have owned a total of 6 lowrance gps units over the years .. I never had this problem with the 300 or 500 series. I have also experimented with zooming in and out as I approach the mark but it still is lagging behind. Any suggestions?