kritter - check the cable housing on your wife's bike. I had a similar problem this past weekend and it was caused by the housing having some rust in it. This would not allow the cable to move freely causing some ghost shifting. It only costs about $5 - $10 to buy new housing and cable to replace it. It also helps if you use brake housing rather than shifter cable housing. It is a little wider and allows the cable to run smoother.

On a side note, this reminded me of why I like sealed cable systems so much.