This worked for me, too. The elevation readings on my Garmin 910XT were completely messed. After reading various forums and threads I was really worried that the barometric sensor on my unit was faulty. Then I read a suggestion about simply washing the unit with warm, soapy water and using a toothbrush and pin to clean the grime (sweat, sunscreen, muck) from the barometric sensor holes. I washed it thoroughly and then blew out the holes with some compressed air. Then I did the diagnostic reading routine ("on" + "enter", then 7x clicks on "mode") and everything seems to be working. Now I get proper elevation readings. I also found this blog post that's worth a read...
http://www.alananna.co.uk/...in-forerunner-910xt/