This happens when you turn the knob master mode selector to fast to "NAV", when the off symbol still showing on the ADI ball. If it happens just turn the knob master mode selector back to "BER" and the back to "NAV".
If it happens during take off, it is because you have wrong airfield in LS. If you look at the gyro compass ring if will be corrected when you accelerate on the runway. For instance if rwy hdg is 27, when you line up your gyro show 265 the computer will correct the deviation to 270. The gyro will jump to 270. But that ofc that is if the computer know where you are.