I can solve this mystery for you as i used to have the same issue there and also at a few other airports.
What it is, is that FSX has a thermal placed in front of that runway which makes your plane gain altitude when you fly thru it.
You can fix it by doing one of the following things.
1. Disable thermal using the tick box in the FSX settings menu in the weather area. I believe from memory the box says disbale thermals and turbulence.
2. The other option is to leave that box unchecked and go into the root FSX folder and scroll down until you find a file, not a folder, called thermaldescriptions.xml or something to that. It does have the word thermals in it. Just rename the file to thermaldescription.off or. bak. This will turn off the thermals in FSX but still allow the turbulence.
If your using a weather program like As2012 or Opus, I would do option 2. This is what i do and it works really well.