Well, as you predicted, 518 was taken out of service for sometime about a few months ago, but was placed back in service as a Siemens test unit with Siemens parts. Probably Siemens getting first dibs for the LIRR to order Chargers to replace the DMs.
Other than that, I am unaware of other times which 518 was taken out of service, except maybe for minor repairs sometime between now and the last post here (almost 16 years ago).
So after updating this post with recent information, and reading the previous comments on it, which one would you prefer on the LIRR: EMD DM30AC or Siemens Charger? (Both have been reported to have numerous problems, but no one's perfect)
"Look down, step over, and watch the gap!" - Dr. John Clarke, the Gap Rap, 2010