Another thought I have that might be worth a try; Try putting some dielectric grease over the connections. Might even try Gardner Bender Ox-Gard Anti-Oxidant Compound in between just the battery connections. I've used both for a garage radio repair that was having some static issues & it has been flawlessly working ever since. Beats spending $50+ replacing the radio & it took just a few minutes to open up to conduct the repair.
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-Oh Hey!