I had exactly the same problem a few years ago, and after weeks of trying to work out the problem, spider mites were indeed the problem. I could actually see them, if I looked really closely at the plant. Following advice, I washed it thoroughly under the shower with soapy water, and then a couple of times a week with water for a few weeks to disrupt the spider mite life cycle. The plant hasn't fully recovered to its original flourishing state even two years on - but it is healthy and stable.
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