Since moving the antenna connection on the back of the unit seems to help from time to time; I would have to guess the connector on the cable most likely has a problem. This could be from corrosion or broken wires that make contact poorly or intermittantly. I would recommend removing the antenna cable from the unit and opening up the connector and giving it a good inspection. You might need to cut the cable back and re-build the connector. Many times you can re-use the connector but if it is too far gone, you can always buy a new connector to put on from your local authorized Furuno dealer. By the sound of it, the odd are, that will reslove the issue. If not then there are other troubleshooting steps you can take.
I should have added I just replaced the antenna (017) a couple of months ago bought from PSI. Unless the factory connector is questionable. What about the possibility of the connector on the unit being bad?????
Well, with a new antenna then it does sound like it is something internal to the unit. It could be as simple as a broken solder joint or complex as a bad GPS receiver module. I would recommend sending the unit into our Factory Service Center (Camas Washington) for evaluation. They have a $100 evaluation fee, but if you repair the unit the amount goes towards the repair so you don't really lose anything. These unit also have an internal battery that should be changed about every 5 years and you you haven't ever changed it, this might be a good chance to have it done while the GPS get repaired.
Dilution of precision (DOP), shows how tight a fix you have based on the number of SATs being picked up and other fixing methods like WAAS and DGPS. Normally with DGPS or WAAS you will have a DOP around 1. If you don't have WAAS or DGPS the DOP will be higher (maybe around 2). The GPS SAT Status screen will show how many SATs your are receiving and what the signal strength is. You need at least 3 greater than 40 to get a GPS lock. Normally most people will have about 6 above 40 and receiving 8 or 9 total SATs.
Don't know what your installation is like or what kind of boat you have but external "noise" or electrical interference of some kind could be hurting you. Be sure to try the unit out with everything else turned off. If the unit ever works normally (even for a few minutes) it's probably not an internal failure. Problems with the unit are not usually intermittent like that.