You might try seeing if the loss of GPS lock happens when you're on a particular comm freq. Spurs from the LO in the comm receiver could be finding their way out the comm antenna and back into the GPS antenna. Only takes a very small signal to disrupt the GPS since they're so close together. Common problem, like cell phones and Bluetooth. Another thing to try is to turn off all other RF equipment, even any form of a receiver, and see it the GPS recovers. If it does, turn them back on one at a time until you find the culprit. Don't run into anything while you're doing all this!