Dillon:
I use FSX and don't know if what I experience is what your describing. Didn't experience this at KLAS, but at some other airports: on approach to KDFW (forgot which runway) the descent rate is constant, then suddenly the plane starts ballooning like crazy until it's a nose-down approach. It sort of does feel like the airport is sinking, and I don't think it's weather-related or anything. Or on finals to runway 5 at LSGG, the approach is at a constant descent rate until, always at the same area, there's ballooning followed by excessive sink rate. Same thing happens on approach to KSNA runway 19R. It's always puzzled me but I never brought it up because no one else did (don't know if we're talking about the same thing though, different sim and all...)
EDIT: I just found out my "problem" is just some default fsx thermals on the approach path defined by the LC, not weather. It can be disabled along with turbulence or the LC file can be edited. Not sure if this is your problem though and you are using fs9 anyway...
Regards
Ron