I just picked this one up yesterday.  It had been sitting for long time but is in excellent condition.  When I first fired up it was all good radio was great and line in sounded good. After about 10 minutes it started with really bad feedback through line in and some popping (both intermitant) when I pull the rca's sometimes it stops but will come back.   When switched back to FM it works flawlessly.   Thanks for any help guys.  

Could also be just oxidized and dirty. Spray small amount deoxit into jacks and using your RCA plugs work them in and out 10-20 times. Hopefully this cures it...

Thank you very much deoxit did the trick,  this box sounds great I'm all smiles!