might make sure your cables are sound and the gound connection is clean and secure first, if it acts the same, then find an auto electric shop, they'll test and repair it .... otherwise, try the swap meet for a good used one
I'll second that. Check cables for internal corrosion and separation. Check all cable connections are clean and tight and all ground straps are present. I had a cable that sulfated (?) inside the insulation and went open. Took a little tracking down as it was an intermittent problem.
