From the sound of things, I think you may have an antenna problem. BTW: I've owned Lowrance GPS system since they first became available. The first one I purchased was DOA, called Lowrance and they overnighted me a new one. I've NEVER had a GPS failure--even during the nastiest storms anyone can imagine. I currently use the Lowrance HDS7, which has performed flawlessly for two years. I have friends that have used Raymarine gear, switched to Garmen after encountering similar problems, and never had a problem since.
First, I would contact Raymarine factory service and see if they have a solution. Do this with the GPS-Plotter in place and talk with them on your cellular telephone while the GPS is running and the problem is present. There are certain reset functions they can walk you through that may provide a solution, especially if it's a software problem. If it is a hardware problem, they can walk you through a series of tests to determine exactly what is taking place, and at that point they'll either provide you with a fix, or have you return it for service.
Hope this helps,