87 Volvo 740 radio. must hit it to work?

my 87 volvo 740 radio, (the turbo model if it helps) must be hit on either the right or left hand side to work. sometimes when i turn on the car, it works, other times it fades in and out. It makes a sound similar to a tv that isnt getting a good signal, that has the black and white fuzzys. Are there any suggestions to get this radio to work all the time without replacing it?