Robot losing and regaining drive power

[PSHRobotics]

During our regional competition we would lose control of the robot, but then after a few seconds we would regain control although it would start a bit sluggish. Now it appeared that it never happened while tethered, but we did not run enough while tethered to be sure.
We would also have matches where we would not see this problem at all, and there was not any noticeable correlation between the matches we did not have the problem versus where we did have the problem.
We aren't necessarily sure what was causing the problem, but it looked like some sort of power loss to me.

As the "coach" who went up with the drivers, I was watching the voltage value on the classmate when we had the problem. There would be no change in voltage when power was applied and there was no movement, but the moment the robot moved the voltage would drop from about 12.5ish to 11.75ish. The voltage, when the robot was functioning normally appeared to be in a range from 11.8 through 12.2.

Any input as to what could be going on with the robot would be appreciated!