Mine has the same problem: I can't arm / disarm from the doors but when I lock / unlock from the tailgate (mine is a wagon) it works fine!
1995 MB E320 Wagon, 42k
1992 Land Rover Range Rover County, 186k
1996 Volvo 960 Wagon, 105k
1996 Ford "Exploder" 2wd Limited, 132k
1982 Volvo 240 GL Diesel, 157k (61k on the gas conv.)