Shifted Layers on Replicator 2
Its not possible to say without seeing the physical machine but I would assume its a mechanical problem. Perhaps the belt or a gear is worn out missing some piece so it slips sometimes. Or the gear its a little loose on its axle. When machine is off you can move the extruder by hand and try to feel if it does so smoothly. 

I would not think its electrical but first thing to do is to detach all cables and put them back, because its easy to do. Second is the dismount stepper motor and check that its gear is attached firmly. If there is a bad cable then flexing and pulling it slightly might escalate the problem so that its always broken, which can be a good thing since intermittent problems are so hard to find.

If its the stepper motor driver electronics then a new driver board is needed, last thing to try unless you have two machines and can borrow a part.

I emailed makerbot support once about another problem, I did get some advice that way.