Does your heart rate monitor work?

Maybe this is a case of you-get-what-you-pay-for, but I bought a heart rate monitor from someone on Craigslist, hoping to save a few bucks. It’s an entry-level monitor, and I’m wondering if the problem it’s giving me is a function of that, or if it’s heart rate monitors in general.

The problem is that the heart rate it reads is erratic. As I’m running, I’ll glance down and see it steady for a while, and then, for a few seconds, it’ll be off by 30 or more BPM. Sometimes it’s higher, sometimes lower, so I think, on average, it comes out. The biggest problem I have with it is when it jumps way up, sometimes as high as 220, so I have no idea what my max heart rate was during my workout.

Is this my particular unit, the model, the brand, or is this just something you have to put up with when trying to monitor your hr? I just changed the batteries in both the transmitter and the receiver, and still have the same problem.