This would suggest that its a "one of" problem possibly caused by condensation which has now cleared, or the previously recorded part of the tape would also be sounding faulty, which you say it isn't. If you play the tape from "before" the bit you know is faulty, and it plays OK and then "not" at the same place again, then either it has recorded badly through the tape possibly sticking (condensation!) or its the actual cassette thats defective.