Interference is an interesting possibility, as is an intermitent battery
connection.....as battery contacts corrode or otherwise make
If you know a Ham nearby, or somehow can ask one for help,
a good resource might have a spectrum analyzer to survey the
RF band of interest and give you an idea if this is in fact the
issue. By taking a reading over a period of time, a ham could
clear this question up.
And remember, Hams love to help others with technical questions
PS..one trick to eliminate interference is to realign, or tune the antenna.
Sometime attenuating the signal works too. In your case, IF IT IS
interference, shortening the antenna might be the trick, as it prevents
overloading the front-end of the receiver. This reduces the performance
of the receiver as well, but increases the odds of near-field reception
which you seek. However, this is the theory. DON'T adjust the
antenna until you know for SURE that this is the reason.