It sounds to me like your throttle levers are somehow being pulled into reverse, which on the B-52 Driver, is the engine cutoff position.

Are you using a Joystick/Yoke/Throttle quadrant that has the ability to pull the throttle levers into reverse? If you do. Try disabling your controller (CTRL+K) and THEN starting the engines. See if they remain running that way. And if they do, you will know it's the throttle controller you are using.

Mark