2 days ago I unrolled my 100 feet extension cord (it was all twisted)... and actually it started charging normally again.
Yesterday I went to the store and had my cord cut to 65 feet (+ another 35 feet cord); still on 12 gauge though, apparently it wasn't possible to have a 10 gauge..
Well anyways, it works now, hoping it stays like that for good.
Of course, I have another question: should I unplug my 120V Voltec charger from the extension cord whenever I am not using it? Or does it matter?