Believe it, or leave it, I've actually solved problems by resolding with actual 60/40 solder on new gear. The reason??? That damned LoHS, (Lead Free.), solder just sucks! So, ya might try looking for what looks like a cold solder joint. (That maybe many, as well as those leftover solder hairs that are conductive.) Another thing to look for? Under voltage electrolytic capacitors. Last month, I bought a Uniden Bearcat 980SSB CB rig on sale. But, I did take care of one big issue that Uniden still has, under voltage electrolytic CAP's. Sure enough, I've found, and swapped out, three of them. At least, these could be places to start at.