I've got one of these, which had much the same problem. In my case the battery was failing. I took the battery apart (fortunately it is screwed together). The cells had begun leaking and the electrolyte had corroded the interconnectors, splitting one in two. A few minutes (VERY careful) work with a good soldering iron and some copper wire fixed it.
For a while at least.
Quite honestly though, it's now on borrowed time and I'll probably just buy a new one when it finally dies irrecoverably.
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