My Northstar 660 often needs a very long time (15min-2 hours!) before it can determine my position. In the "satellite" menu I can see that the GPS has found a number (5-7) satellites but the bars are all blue, In the top menu (dialogue menu) it says "acuiring" all the time until the bars are green and the position is set. This problem especially occurs when it was a long time (1>week) since I used the GPS whereas it works just normal if I used it the day before.

SOURCE: Gps loses signal

It would be good to make sure that the 951 software is up to date. The most recent software upgrade includes new code required due to changes that have been made to the GPS system. You can download the software for free from the Northstar web site but it requires a loader cable, power supply and a Windows based PC to perform the update. Most Noprthstar dealers know how to do this but they will charge for their time. The GPS antenna should be mounted where it has a 360 degree uninhibited view of the sky right down to the horizon to get best performance. Basicially any conductive material including pigmented paint, tinted glass or wet canvas can block the signal. Inspect the GPS coaxial cable and it's connections for any signs of corrosion or contamination. Sometimes just removing and reinstalling the connections is enough to remove contamination and get better performance. If in doubt concerning the coax and it's connections, temporaily substitute a new coax assy by-passing the existing cable to see if this makes a difference. You can monitor changes from the GPS status page. See the user manual for information concerning how to understand the numbers being presented. You should be tracking at least 4 SATs with signal levels higher than 30. Generally 95% of all GPS signal issues are related to the coax or its connections.
coax assy to v