I have the Edge 705 and am having a similar problem, but I'm very surprised about how it manifests itself.
Last Thursday, I decided to explore the course of a century tour that is coming up soon here in Utah. I ended up driving the first 25 miles, parking and then biking the next 50 miles (an out and back), then drove the remaining 25 miles. When I look at the history on my Edge, I see all three "rides", including the two laps (out and then back) that made up the 50 mile ride.
When I try to upload the rides, however, it seems I can only access the two driving portions of my day. The 50 mile bike ride doesn't get uploaded and I can't see a third .tcx file on the Garmin either. Unfortunately, it was the 50 mile portion that I was most interested in reviewing because it had actual performance data.
Do you think this is the same issue? I don't have that many rides stored on my Garmin, but if it was running out of memory, wouldn't it overwrite *old* events, rather than lose the data for a "current" event?