Hi Paul, I think you'll find you need a new antenna. I've had the same problem twice!! Once with the older LGC-2000 and once with the LGC-3000. After all the trouble shooting it was always a new antenna required. Lowrance said if it was just outside the warranty period they would still send a new antenna for free as it is a common fault. Unfortunately it was a unit bought from the USA, which they knew as soon as I gave them the serial number of it. They did however sell me a new antenna at cost, which was about $150.00 cheaper than retail.