Hi CoolCarbon:
Not sure offhand, could be a variety of things.
The physical connection between the antenna on my SP III and the body of the GPSR itself began to show signs of wear after about 2 years of use on the moto - the antenna would not stay upright, it kind of flopped down. I thought I had this problem solved by putting a little rubber O-ring around the antenna base, to increase the friction between the antenna and the GPSR case, but I later found out that the electrical connection between the antenna and the GPSR was not as "tight" as it was when the unit was new.
Eventually I switched over to using an external antenna. I didn't bother to run the antenna up onto the roof of the car, I just placed it on the dashboard, up at the front and centre. This gave me much better satellite reception (faster pickup, more satellites all the time), therefore fewer "Satellite Reception Lost" messages.
A reasonable quality, small external antenna costs about the same amount of money (USD $40) as a replacement Garmin mast antenna, so you might want to think about going this route, if the electrical connection from your antenna is not first-class.
Before doing this, though, try taking a Q-Tip type cotton swab, dipping it in isopropyl alcohol, and cleaning the inside of the BNC connector on the back of the GPSR, and cleaning the BNC connecter on the antenna itself. See if that makes any difference. The photo below shows where to clean - it is a photo of a 296, but the connector on the SP III is the same.
After having used an external antenna, though, I'm pretty much hooked on them - I even use an external antenna when I have the GPSR's on the motorcycle. It does make a huge difference.
Let us know how it turns out...
PanEuropean