[MGPAX]

The reason the voltage is dropping so fast and then climbing back up again is because the batteries are being over taxed. You're not draining the batteries down and recharging them, you're drawing them down and then they are recovering.
Normal battery recharge takes time...it doesn't happen in minutes...These are not Lithium Polymer batteries..and they take at least an hour to fully charge.
The same way as if your car doesn't start and you keep the starter turning for 8-10 seconds. Only this is doing that on a regular basis.
Which (I've always understood) is murder on the batteries...