Here are some other suggestions:
It is abnormal for a front derailleur to go out of adjustment that quick -- new cables are the exception cause they stretch for a while.
Can you hear or see the derailleur rubbing on the chain when shifted to the middle chain ring ? If so, it does need to be adjusted.
It is possible that the chain has a bad link which "kinks" under the lighter load that the middle chain ring places on it as compared to the large chain ring.
You may have bent tooth on the chain ring, although that can't explain why it only starts to happen after a few hundred miles.
Most of the these 'difficult' to solve problems are caused by multiple things that by themselves are not problems, but added together stack up to be a bigger problem.
It is a very, very good idea to learn how to make adjustments on your bike yourself. I quick google search on derailleur adjustments should give you a starting point. Also, you can download tech documents from the Shimano USA website.