Is my HRM broken?

I've had some problems with my HRM lately. Sometimes when I glance down at my watch, and I feel like I'm running full speed, heart beating, sweat running down my face, it shows my heart rate at 120? Usually when I feel like that it's up at 160. I was going to buy a new one with my next pay, I love Suunto watches, but saw this white polar and fell in love! Now I can't decide :)