loss of range on airport extreme

about nine months ago got an airport extreme and it has been working perfectly til now,,, imac has always been about 30 feet away in adjacent room and got good connectivity with 20+ mbps download on wifi... suddenly now less than 0.2 mbps. Moved the imac into the same room as extreme and 20+ mbps. upgraded firmware plus factory reset, installed ML on imac and problem continues - - - also have loss of range with Air, ipad and iphone.... airport express will not connect to extreme now either (upgraded firmware, factory reset, etc)