I'm having a similar problem with a Garmin HR strap. I changed the battery a few days ago, but the problem persists. Today I opened the battery compartment after a ride. There was moisture inside the case, but not inside the actual battery compartment that has a rubber seal. I removed the moisture, adjusted the battery contacts to make better contact with the battery, and placed a paper shim under the cover to hold the battery tighter against the contacts. I'll see how it works tomorrow.