Voltage Drop issues

[mypie4050]

When testing our robot recently, we encountered an issue where we received a significant drop in battery voltage, and therefore a drop in performance and a loss of control. This occurred whenever driving the robot on carpeted floor, particularly when turning and when crossing defenses. We experienced the problem on two particular occasions. The first was after a prolonged period of driving (over defenses and on carpeted floor). The victors controlling our drive train were rather warm. We assumed they had overheated.
The second occured on our first test after enabling the braking function on the Victors. To sum it up:

The Robot:
6 pneumatic wheels from AndyMark on the AM14U3 chassis
4 CIM motors, 2 on each of 2 toughbox minis with a 10.71:1 gear ratio
1 Victor SP controlling each pair of CIMs.
2 other motors each controlled by one Talon SRX via CAN
Weighs roughly 80 pounds

The problem:
Major power drop when turning or driving over defenses
Drop from 12 to approximately 6-8 volts

Potential Causes:
Pneumatic wheels don't have enough air
Braking feature of Victors eating up a ton of power
Victors being power hogs
Victors overheating
Faulty Wiring
Faulty Victors
Extreme friction caused by pneumatic wheels
[Edit] Two motors on one Victor

We know it isn't:
The batteries; did the same thing to 3 of them, at least one is new this year
The CIMs spinning in opposite directions; There is no power drop when the wheels are off the ground.

Any ideas as to what the problem might be? We try removing the braking feature on Monday to see if that is the issue.

Edit: Found out it's actually illegal to run two motors into one controller. Will be fixing the problem shortly