Frustrations with my HRM

I have a Nike Imara Heart Rate Monitor/Watch Set, and I really do love it......dunno where I would be without it. I just get super frustrated when my heart rate goes ABOVE 180, the HRM doesn't register it and throws me heart rate down into double digits (55 BPM or some junk like that). This typically happens when I am going for the gold on the Stair Climbing Machine at the gym. Is there a way to manually calculate the calories burned based on your age/weight (29/142lbs), HR (180 BPM) and how long you were exercising(35 Min)? I hate when I have to guess how many calories I burn. I assume it's more because my heart is working harder, but I like to KNOW for sure!

Thank you!