I helped a friend of mine with a troublesome Garmin 128 that had a similar problem that you describe. It turned out that his original GPS antenna coax had an internal break. I got a replacement antenna from GPSBitz who were very helpful. I actually bought one of their standard antenna's which are waterproof and gutted the original Garmin marine antenna and fitted the new antenna inside the old pod.......saved about £40! Colin at GPSBitz told me to check the GPS unit by testing the voltage across the inner and outer of the antenna socket in the back, you should get between 3 - 5 volts. If this checks out ok, then it is most likely an antenna problem. I didn't have to do this in the end because I had a spare antenna for my own handheld Garmin unit, when I plugged this into my friends GPS it sprang into life........
-------------------- I came into this world with nothing..........so far I've managed to hang on to most of it