My 1.107 Belgian Dark Strong went nuts like that, lost at least 1/2 gallon then the fermentation really stalled aroun 1.035....I have since gone to FermCapAT and am getting ready to do another big beer to test it.
Something is always fermenting....
"It's Bahl Hornin'"
Kegged: Bourbon Barrel Imperial Stout
On Deck: German Lager