Interference question

[radio nut]

I just got a suitcase Zenith trans oceanic radio...when I first picked it up it received pretty good. After I recapped it the performance went way up. In my area there is some sort of on and off interference that does not have any pattern to it but reminds me of a fluorescent light with a bad ballast. On AM when the noise kicks in I can get only 2 local stations......the rest of the band is loud hum.....even on low volume. Is there anything I can do? When the hum is gone sometimes AM is clear and then other times background crackle .......