Just want to chime in on the HR readings…
I get the opportunity to sit with the service team here almost daily and talk about everything related to end user experience.
The OP’s issue is the most common by far - and isn’t necessarily limited to Polar devices.
Generally when you are looking at HRM’s you will have two types of transmitters
A typical transmitter like this that consists of a 6 inch rubber type transmitter secured by an elastic strap 
A textile/fabric transmitter like this
Most brands offer the same type of two transmitters. The major difference is that the electrodes on the second transmitter(in our case Wearlink) are built into the strap itself. So the points that have to make good contact with your skin are housed in the textile fabric. The advantage of this design is comfort.
On the first design, the electrodes are on the back of the rubber itself with two grooved areas.
The issue you describe most often happens with the newer textile transmitters. Mostly all transmitters we get back for service with this issue have one thing in common - they are DIRTY.
The first rubber style transmitter was easy to keep clean - everything was exposed. So a wipe down with a damp towel or a rinse down could clean it in a second.
The second transmitter style, since the electrodes are built into the fabric require a little bit more maintenence. The center transmitter piece should be removed from the strap so it doesn’t constantly search for HR signals, and the strap itself has to be washed regularly(in a washing machine). This doesn’t always happen, so eventually it’ll stard building up sweat, corroding the contacts…etc… Soon enough - things start acting funny.
Also - the strap itself does eventually wear(almost like the elastic on a set of bibs would). Usually happens around 1-2 years depending on use. This is a consumable part and we offer them separately for purchase when it does go out($15.00)
We are working really hard to communicate the maintenence of the strap to make sure the user experience 6 months, 1 year down the line is still good! So if you have any feedback or anything that would help us get the word out about maintaining the strap please drop us an email @ customer.feedback@polar.fi
Thanks!
Chris