This may seem like a silly question, but what is the temperature in your garage when you train? I bet you are covered up in sweats during the winter time. Also, if possible list your squat routine. When I squat, I train like a bodybuilder typically performing 5 sets x 15,12,10,8,6 reps. It seems that a lot of guys around here train with 5 sets of 3 to 5 reps.
I just about did. As your muscles wear down you lean back to keep the stone up. When you lean back, the stone sits on your chest and you can't breathe. The less you can breathe the harder it gets...