GPS not able to lock (SNR often 0, sometime in thee 5-10 range)

Hi, I have yet another Magellan 700 that appears to have issues trying to lock to a signal. Even after resetting the GPS, manually setting the location, and waiting for 5-10 minutes for lock, it only works about 20% of the time. Even when it does find the location, it immediate loses it if I move more than a few feet. The AZ/EL/time appear to have real values (not sure how accurate they are, but they're non-zero). If lock wasn't achieved (80% of the time) the SNR shows 0. If lock was met (20% of the time), and then I move, the lock is lost (red icon, then black), a few of the satellites show a really low (values in the 5-10 range) SNR and the rest show 0. I've tried with 2 different antennas and it doesn't seem to matter. In fact, disconnecting the antenna doesn't seem to affect the SNR, as displayed on the GPS status screen. I sort of suspect the circuitry that RF modulates or amplifies the incoming signal, but I really don't know much about the internal technical details. Any suggestions? Thanks, --ken PS. Does anyone know how to interpret the last few columns of the GPS status (rev 4.0 software) screen?