abs problems on '94 F-250

I have a '94 F-250 that the ABS light will be on or off with no regularity. Sometimes when stopped, the brake pedal will go down to the floor. When it does this, it seems to go down in levels. Also when we tow our fifth wheel trailer-there are times that I feel that brakes are working at 50%. It's very concerning. Of course, the brakes never act up when I have somebody look at them....Any suggestions?