Polar RS800CX - GPS accuracy?

Hey all,

A couple years ago I picked up a used Polar RS800CX, and used it exclusively with a foot pod for running. It seemed accurate enough right out of the box, and I could always re-calibrate it on a track if I needed to. I was always, however, a little jealous of my friend's Garmin, because he could quickly and easily share his runs, it mapped them, all those cool little things.

Well, this past fall, my foot pod disappeared. Personally, I suspect the toddler put it somewhere, but I can't get him to admit to it. 

So I purchased the GPS unit as a replacement. And I've been using it, and finding it somewhat inconsistent. I don't know if it is a problem with my particular GPS unit, with Polar GPSs in general, or with all GPSs.

For example, I've been running the same out-and-back most mornings for the past week or so. Typically it comes up as 2.1 miles. This past Friday, it came up as 2.4 miles. That's a difference of like 15% - that's not within my margin of error. I knew something was wrong when it told me I did a 6:25 first mile - I don't think I could do that without a jet pack.

Thoughts? Recommendations?

Thanks!

stephan