That is typical of most GPS receivers. I have 2 in front of me now connected to 2 different Arduino's and position data shifts in about a 300 meter circle centered on my apartment, depending on the number of birds (satellites) that are above me. The more sat's the better my position accuracy. I watch the Altitude data tell me that my house is moving about 30 meters up and down as well. Fortunately I don't need to know my position... I know where I live, The GPS receivers were the easiest way to get accurate time without either using the internet NTP server or the NIST WWVB service from Boulder Co. as it isn't available really except at sunrise and sunset due to propagation issues with the extremely long wave (60 KHz) used to transmit the time codes. The reason for the receiver issues that force you to be near a window is simple most building materials are opaque to the 1.7 GHz signal transmitted by the satellites and thus the signal to your receiver is a great deal weaker than it needs to be for proper operation Storms also cause similar effects due to the water vapor in the storm clouds. My receivers are two, a U-Blox-6 (35 X 35 mm) I bought from Electrodragon and a Skylabs SKM53 ( 20 X 27) mm I bought for $26.00 from Ebay. What you might do is buy a receiver with a separate antenna there is usually 3 to 5 meters of cable from antenna to receiver or you can put a 3 conductor cable on your receiver, put the receiver in a water tight enclosure and remote it... 5 meters of 3 conductor shielded cable would work very well. The third option is to use a transmitter and a controller (Mini-Pro?) to remote the receiver wirelessly. I hope this helps.
Bob