Got a Garmin for Christmas (thanks me

) It has a heart rate monitor which I don't really use for training, I've just been keeping an eye on it mostly because it's neat. It seems to operate consistently. Today I went out for an easy 5 mile run on the roads. 8:45ish is my current easy pace. I steadily increased my speed throughout the run. My heart rate for most of the run was around 165, then it sped up and fluctuated between 165 and 180 for the last couple miles. For the last half mile I decided to pick it up and run hard the rest of the way. Strangely, my heart rate dropped down to below 150 (according to my device) as my pace quickened to under a 7 min. pace! It held steady in the 140 range until I sprinted the last 50 meters or so when it skyrocketed up to the 180's again. This is so strange to me that I can't stop staring at the graphs! It doesn't seem like the device malfunctioned. Does anyone experience a similar phenomenon?