Hi Joel,
The topic was brought up some time ago but I’d still like to ask back something: which type of HR monitor were you using? Are you sure that the measurement itself is correct?
I’ve seen this kind of deviation while running (very high HRs) on Polar and Suunto products with different sensors. Usually the measurement settled after a few minutes. Today I believe that it could be caused by the cable of my headphone or one specific running shirt. I know that the latter sounds esoteric but there might be surface treatments to the fabric (anti-sweat?) with effect on the data link between sensor and monitor.
Best regards, Jörg