The way I would do this is with a persistent effect and an issue order effect.
First create an issue order effect, then set the effect "Effect: Ability" field to Move, set "Effect: Player" to source, set "Effect: Unit" to source and finally set "Target: Target" to "(None)Target Point".
After that create a persistent effect, this effect will handle the location of the target point where move command is given. In the persisten effect set the "Period: Count" to 1. Then add three durations of 0.0 seconds in the "Effect Period: Duration" field. Then in the "Effect: Period Effects" add the issue order effect you created previously. After that go to the "Effects: Periodic Offsets" and add three offsets, this will be used to set the direction in which the move command is given. The offsets have three parameters (X, Y, Z), in one of the offsets put (X: 0.2, Y: -1.0, Z: 0.0), in another put (X: -0.2, Y: -1.0, Z: 0.0) and in the final one put (X: 0.0, Y: -1.0, Z: 0.0). After that go to the "Effect: Random Offset" and "Effect: Random Period" fields and set both to enabled. Finally go the "Target: Location Offset - Start" field and set it to "(None)Origin Point". Then add the persistent effect as periodic effect to the behaviour that makes the ship drift.
The persistent with these parameters will choose at random one of the three offsets once and execute the move command, taking the original position of the unit with the behaviour and issue that unit the move command to the target point with one of the offsets. The -1 in Y parameter of the offsets, sets the target point for issue order effect, to be to a distance of 1 ahead of the initial position of the unit with the behaviour, the -0.2 or 0.2 in the X parameter of the offsets, will move the point set the by the Y parameter value slightly to the left or the right of the unit with the behaviour and the offset with a parameter of X = 0 and Y = -1 will order the unit the unit to go forward by a distance of 1.
I hope you find this helpful, tweak the values of the offsets in the persistent to achieve the desired effect.
0.954959053685168
The way I would do this is with a persistent effect and an issue order effect.
First create an issue order effect, then set the effect "Effect: Ability" field to Move, set "Effect: Player" to source, set "Effect: Unit" to source and finally set "Target: Target" to "(None)Target Point".
After that create a persistent effect, this effect will handle the location of the target point where move command is given. In the persisten effect set the "Period: Count" to 1. Then add three durations of 0.0 seconds in the "Effect Period: Duration" field. Then in the "Effect: Period Effects" add the issue order effect you created previously. After that go to the "Effects: Periodic Offsets" and add three offsets, this will be used to set the direction in which the move command is given. The offsets have three parameters (X, Y, Z), in one of the offsets put (X: 0.2, Y: -1.0, Z: 0.0), in another put (X: -0.2, Y: -1.0, Z: 0.0) and in the final one put (X: 0.0, Y: -1.0, Z: 0.0). After that go to the "Effect: Random Offset" and "Effect: Random Period" fields and set both to enabled. Finally go the "Target: Location Offset - Start" field and set it to "(None)Origin Point". Then add the persistent effect as periodic effect to the behaviour that makes the ship drift.
The persistent with these parameters will choose at random one of the three offsets once and execute the move command, taking the original position of the unit with the behaviour and issue that unit the move command to the target point with one of the offsets. The -1 in Y parameter of the offsets, sets the target point for issue order effect, to be to a distance of 1 ahead of the initial position of the unit with the behaviour, the -0.2 or 0.2 in the X parameter of the offsets, will move the point set the by the Y parameter value slightly to the left or the right of the unit with the behaviour and the offset with a parameter of X = 0 and Y = -1 will order the unit the unit to go forward by a distance of 1.
I hope you find this helpful, tweak the values of the offsets in the persistent to achieve the desired effect.