Question about Gauntlet mode

I know I can't use all these new cards in the Shandalar game, so I've been using the Gauntlet to play multiple opponents without having to return to the selection screen. One thing I noticed is that sometimes the game will end early when the computer throws in the towel, realizing that it's tapped out and has no answer to *insert lethal combat attack here*. Now while I don't mind not having to tell it I don't want to activate any fast effects or sometimes even declaring a combat, why does it end the game early the other way? Twice I've had the game tell me I've lost before I was done with a phase. I forget what happened the first time, but the second time I had 3 life and a Drain Life in hand, and as soon as I activated Cabal Coffers to produce 5 black mana, the game ended because I lost. What's up with that? Is there any way to tell the game not to count me out until I've 100% fulfilled a losing condition?