I was going to say...
that it could be your battery...but you made a brief mention you have a bunch of batteries...so I am assuming it shows the same symptoms regardless which battery you use,
and as well...it could have some little to do with your charge cable...

Buttttt....
all things considered ...if it is a n900 you messed with originally ...fixing the gsm...
well ...god ...anything ...right down to the bme may be wonky now.

How long after the repair you did...did the symptoms first start appearing?
Right away? or did it take some time to manifest?

If you can't quite fix it...saying the battery still wishes to get too much unbalanced incoming charge , thus making the chemicals in said battery a serious threat...
If you do get the bme even quasi stabilized ...
then you could perhaps safely use a dummy battery with a proper regulator and use it hooked permanently to usb, to supply it power.
a micro desktop maemo pc ...essentially...