[Scott Johnston]

I warmed up for 25 minutes and then lapped my watch at 25 minutes right as I made my final speed and incline adjustments.

If I understand you correctly, this might be the source of the large drift from 130-137 in the first half of the test.  You started your test just as you made an adjustment to the pace and grade.  You should be finding a pace and grade near the second half of the warm-up, where your HR stabilizes.  Then start the watch.

Scott