motor cutting out?????

[Rhino]

ok here is the situation at about 55-60 mph and starting around 2700rpms my truck will start cutting out and will not go any faster, but if i put it in 2nd gear it will runn fin all the way up to 4000 rpms and easily cruise at 55 mph....so my question is (having little knowledge of transmissions) is this a tranny prob or a motor prob....i have done about as much as i can to the motor filters, wires, cap and rotor and it still does this it is a 305 and a 400 tranny.......thanks for the help....