I'm sure you've tried replacing the antenna already, if not, DO IT! Is this problem on the entire fleet? Make sure the four coax cables are connected to the back of the computer and antenna correctly. They should be color-coded, too. Then get a megger and meg each wire out, (DISCONNECTED FROM EVERYTHING!!!)
If this all checks good, and you still can't find a problem, get the engineering dept. involved. I had these same problems before when I worked install many moons ago.