Garmin 620 race predictor

Late last year I got a new Garmin 620 running watch.  It has cool features like measuring ground contact time, vertical oscillation and step cadence.  It also 'predicts' VO^2 max and also has a race prediction algorithm that estimates your potential in a few race distances.  Previously I've reviewed how the VO^2 max varies over time, so today I can at least start reviewing the accuracy of the race predictor.  Here's what my watch thinks I can run the following distances in: 

I'm finally starting to do some running races this spring so I can make some comparisons. I ran in a 10k in Sisters, OR last Friday, and two days later, I ran a 5k in Bend.  The 10k was on trail and although it was not extremely hilly, it was pretty technical trail run, with lots of corners and river bottom like rocks.  So my final time was 39:16. I was happy with it even though my watch didn't think I was reaching my potential.  I'll chaulk it up to the technical trail.

Then on Sunday (today) I ran a 5k mostly road run with very little hills although it wasn't dead flat.  I also think the course was about 0.1km short, at least according to my watch.  My time was 17:44!  Look that's only 1 second off from my watch prediction!  I know the course may have been a bit short, but I found it pretty funny anyway.  Also, I've never run a sub 2:50 marathon but I think I have it in me! My 1/2 marathon PR is also very close to 1:21:15; only about 1 minute slower. 

So far my only complaint with my Garmin 620 is the heart rate often spikes to maximum for the first 10-15 minutes of my runs, a common problem.  I'm pretty frustrated by it, so if anyone has any ideas let me know.  I've tried a few things, but so far nothing seems to solve the problem all the time, especially on cold / dry days.