[Dave]

I would try using a hot air torch to heat your components one at the time until you find an area in the radio that is affected by the heat. You may be able to narrow down your search a little quicker that way. Check your solder joints and also any connections to the chassis (IF cans, tuning condensor, rivets, etc.) that may be loose or not propperly isolated, whichever the case may be.
Dave