odd remote

[scas]

mk6 transit, ive had a few off them. my latest has a problem which ive never had on any of the others. the key fob will lock/unlock the van a million times from up to nearly a hundred feet when the vans cold, as it warms up the distance becomes less until its about 10 foot when ive been driving for about 5 mins. once its properly warmed up the only way to lock it is sticking the fob to the drivers window.
i changed the battery in it obviously and programed a different fob and its still the same, so it must be something in the van, any ideas?? its not the end of the world ,just a bit annoying.