Is it a 'voltage cut-off' issue?

played with the 528 today, I usually only run 6min. cause that's how long races are, anyway Played until it hit the cut-off (3.0v per cell)(sedan, rubber,17.5, 2 cell)(street asphalt) it went really soft, lost "boost", went into what I'll call "You are done Dude, head for the pits" mode. Fresh charge, back in the fast lane!