My cardiosensor began showing 220-250 bpm. I'm reading some forums but haven't found the solution.
I replaced the battery in Garmin - no changes, then replaced the battery in the sensor - no changes. Most likely, the problem was in the strap, and it started after the last wash. I tried to attach the fingers to the contacts of the sensor (without strap), and voila! normal pulse. I have already ordered a simple chinese strap.
But because of curiosity I took off the strap black matte layer. Now, when the buttons is touched to the body, everything works fine. I put a conductive gel for better contact.
So, this is an excellent and quick solution if your strap spoiled, and you are waiting for a new one.