Aloe vera gel works really well for me as an electrode gel. The pure stuff, without added oily moisturizers etc (which can interfere with contact).
I have a Garmin and don't know much about the peculiarities of Polar HRMs, but in mine, intermittent readings combined with max-out spikes happen when contact is poor. Cold weather (i.e. not much sweat) is a big culprit there, and that's where the gel helps a lot. But a sports bra that puts pressure on the strap in the wrong places can get in the way too, and so can too much sweat on very hot days.
If you're in a deep tuck on your bike it can also force the electrodes away from your body. The instructions on my Garmin say that some cyclists get better contact with the electrodes on their back.
I don't think you'd be getting the spikes if the battery was low. Easy way to check for a weak battery in a known good unit is to check the range of the transmitter. Put the strap on, hold the receiving unit close to your chest, and wait until you have a good reading. Then gradually move the unit to arm's length and see whether you still have a reading. Next, turn the unit off and on and see whether it will acquire a signal when it's held at arm's length (or a foot or so beyond arm's length). If it won't, or if it's particularly slow about it, then your transmitter battery is probably weak. Otherwise it's something else.
Speed comes from what you put behind you. - Judi Ketteler