If your Replicator 2 is developing sporadic jumping in the X-axis, it's very likely a short in the X-axis stepper motor cable. This has been reported by a good handful of people on the Makebot Operator Group on GoogleGroups.
Basically, what's happening is that Makerbot used IDC flat ribbon cables and also used roll pins in the design of the R2. The IDC cables are not designed for repeated flex applications, and the edges on the roll pins also were chafing at the cable. The resulting open or short would cause the stepper motors to no longer step correctly, causing the carriage to jitter and lose their X-alignment.