ILS problem in FSX

Hi, It's a while since i last posted anything here, apologies if i get it wrong!I'm flying mainly PMDG 747 and MD11, when I fly an ILS approach, I like to fly it manually, now I can follow the localiser fine up to a point between 1000ft and 500ft from the runway, when the localizer beam suddenly veer's off to one side. Even in good visibility you can see your lined up with the runway, before the needle veers, so it's not that i'm off course. This is making low visibility landings almost impossible though.This seems to be happening at any airport, not specific ones, if that helps any.Thanks for any help.David.