Di2 misbehaving

My Di2 has started to act up. Full battery but will only shift when it wants (both ends). The lights also just come on and off as they please while I'm riding.
Another strange thing about it is that it won't work at all when the bike is upside down, only when it's upright. Could this point to an issue with the battery itself which is in the seatpost?
Another strange thing about it is that it won't work at all when the bike is upside down, only when it's upright. Could this point to an issue with the battery itself which is in the seatpost?
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PP
You think replacing the entire groupset with a completely different system is a valid suggestion, Nick?
He probably has a spare one in his 40kg backpack