Bad conctats...? Try to use some contact cleaner for copper and brass.
Here's an intersting mix i'd tried in my Vintage Radio, that you can mix at home:
Oleic acid mixed with standard Coleman Fuel. Oleic acid is an organic acid found in things like cocoa and olive oil and not considered dangerous. It is used for many things in industry ranging from lubricants to cosmetics to food flavorings.
My recommended mix with the Coleman fuel (naptha) or regular lighter fluid is 5%. You may also add acetone to speed up the drying time.

Btw, here's an interesting link you may find useful:
http://www.justradios.com/safetytips.html