E36 325TDS wobble at idle

I own a 1994 325TDS Auto with 88k miles on it. It has been running well but recently it's developed a natsy wobble from the engine at idle.
There's no sign of a problem once moving, performance is fine. The air filter and fuel filter have recently been replaced, I've run injector cleaner through it, a local non-specialist mechanic has looked at it all to no avail. This has come on gradually, a while ago it gave an occasional wobble, now it's continuous. It's not running irregularly it's just got a rythmic wobble--a bit like a petrol engine that's running too rich, only worse.
Can anyone suggest the most likely cause or suggest what I should investigate and how?
I'd be grateful for any assistance anyone can give.
Howard Taylor
(Chandler's Ford, UK)
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