This, from the Polar website:
Please note: Static electricity in clothing or a flapping shirt can cause electrical interference, so some items of clothing, i.e. man-made fibers, can also be the cause. Please try wetting the t-shirt in the area where the transmitter is.
This used to drive me nuts, now I make sure that jerseys are washed with fabric softener and hung to dry, never tossed into the dryer.