brake pedal has a mind of its own

[wep0520]

Hey all. I just bought an 94 f350 crew cab with a 7.3 in it. The truck has a brake issue but for 1000 bucks i wasnt going to complain.
Here is the problem, when you barely touch the brake pedal it will go down on its own and stop the truck and stay there until its pulled back up. A sugestion to put a bungi cord on pedal to pull it back up woud work and it does but.... A few times at a stop light the pedal felt like someone was pmping it, pressure and release, preasure and release on its own. The truck stps good sometimes and other times it takes more foot presure like manual brakes but suddenly power kicks in and pedal pressure drops. Has anyone run into this problem before The previous owner replaced the booster and the vacume pump seems to be working fine.