RF interference from the PC or the power supply. I had an identical experience to yours. I picked out a nice spot on the rear shelf for the GPS, tested it with the laptop and it worked perfectly, always at least 7 sats and often more.
Installed the PC underneath the shelf. No lock. Didn't work. Had to move the GPS to the front of the car. Found out that when I swapped the screen to a Xenarc, it also interfered with the GPS signal!
Basically, anything in the 1.1 ghz band can potentially intefere with the GPS receiver.