We are having the same problem with our Raymarine c80 chartplotter. I already tried replacing the small battery in the antenna in it. I'm still having intermittent issues. for my next trick I'm going to try replacing the small antenna which I believe is a few hundred dollars. many of the newer chartplotters have a GPS built in but I really don't feel like spending thousands of dollars chartplotter that should be able to keep a fix. since I spend a lot of time in shallow water I keep a spare handheld GPS / chartplotter on just in case I need to fix which is often. please let us know how you make out solving your issue.
