So I drew my normal route on ridewithgps, downloaded the tcx file as suggested by them to be best suited for Edge 500, loaded the tcx file into the device as a course. All working just fine. But when I'm riding, every 30sec/1min or so the Edge will beep "off course" and couple seconds later beep "course found". It gets annoying really quick. I'm not the best bike handler out there but I don't think 2ft side-to-side movement should be considered "off course"...What could be the problem here? bad tcx file?

If you're riding where you don't have full line-of-site to the satellites -- under dense tree cover, in a canyon, among skyscrapers, etc. -- then the 500 won't have as good a fix on your location and may calculate that you're away from your route.

good point - I should have said that I'm just riding in normal suburb 2-lane paved road, with trees on the side but nowhere near "covered" let alone "dense", and when I'm riding through downtown areas, most of them have buildings no taller than 5 stories on either side.

maybe I'm asking too much for an old device like the 500, but how "away" could I be? I'm on a bike, I don't teleport half a block in a second, especially when the 500 knows how fast I'm going from the speed sensor besides GPS-based calculation.

It could be that the file has some bad points in it. If one of those points is a bit out of line, say it's in the middle of the block while you're riding on the street, the 500 would think you were supposed to turn and you didn't. Or it could be the 500 getting poor reception.

I'd start by making sure the 500 is set to record every second, then zooming in on the track it records and looking to see whether it thinks you were where you really were. That should tell you whether it's the device or the file.

It is exactly what you experience that convinced me to get an 800. The problem I had with the 500 is that when you get 'off course', it doesn't help you get back. In unfamiliar territory, that's just as good as being lost. In addition, it always told me to turn, right when I should or when it was unsafe to do so. No advanced warning. Maybe I just don't know how to use the silly thing. The map capability of the 800 with turn-by-turn directions is well worth the additional cost.

The really annoying thing is when you upload your ride and you find the parts where it thinks you were off course are deleted.
See the north east corner of this ride, I think it was the last time I used the courses function.
https://connect.garmin.com/modern/activity/123576843

Although i agree that course navigation is not seamless, the error shown here is a no signal example, not an off course problem. The 3.30 firmware appears more robust now with course navigation. Also, for better turn notification, use bikeroutetoaster to create a tcx file with 300 ft turn alerts from the original ridewithgps file. It is much more useful and even allows adjusting the virtual partner.