I just went through the pain and suffering of never knowing if my unit would lock on to and hold satellite signals or not. I didn't run Garmin but the symptoms were the same. Finally brought the unit head into an electronics shop, used one of their GPS antennas, and promptly got a good solid signal.
I would check:
1)) cord---if it was fished through tight spots, sometimes the connectors get stretched and abused and you'll get intermittent satellite signals
2)) antenna--- was it too close to your radar? Did another boat pass by and fry it in a marina? Did water get in to the connecting pins?
My antenna was mounted on the top of my wheelhouse roof, approx. 6 ft away from the radar. There was an outside chance the radar fried it, so I purchased a two foot long Shakespeare mount and the new antenna now sits 6" above the radar---small price to pay to at least keep the potential radar impact on the GPS antenna out of the equation
good luck in the trouble-shoot