Navigation system doesn't know where I am

Recently I've had several occasions to drive around in the hilly residential areas here on the SF Bay Area Peninsula. I'm finding the Navigation system gets "lost" quite often. The system will think I am one or two blocks away from where I actually am, and will therefore give bad directions. This only happens on twisty roads in areas where there are lots of trees, so I'm pretty sure it is just a matter of the satellite signal not being strong enough for the receiver.

What is strange is that I've had this car since before 2004 and rarely experienced this issue. I am guessing that the recent cloudy weather in the Bay Area is another factor that exacerbated it, but it does feel like the performance of the my GPS system is deteriorating. I never remember it being this bad, where about a third of the time I'm in the hills it is lost.

I made sure there's nothing blocking the gps antenna inside the dash behind the MFD.

Just curious if anyone else in the Bay Area or elsewhere is experiencing reduced GPS performance recently. At first I wondered if the government had re-enabled the "selective availability" feature that reduces GPS precision, but I don't see anything about that on news searches.