FWIS, my Garmin with the plastic strap also recently started giving very high vaues for the first 5-10 minutes of a ride. This is spordic, and MAY be related to putting the strap on to long before a ride (i.e., long enough for the saliva/water I put on it to dry out). It seems to be accurate, once I've worked up a sweat.

Here's a typical example: http://app.strava.com/activities/137022649/analysis

I can assure you my HR Did NOT spike over 200 at the beginning of the ride. As you can see from teh rest of teh ride, even pushing up a 20% grade, My max HR is in the low 170s.