300SE - rough running

Lately, my '90 300 SE has started to act a little strangely.
No problems getting it started and on the move, everything seems OK. Once
it warms up however it sometimes starts to show a tendency to run roughly up to 1,500 rpm. It does not happen every time, but often enough to be annoying and on a couple of occasions it has stalled.
It only appears to be happening under load - if I disengage the transmission it will rev quite freely if I gently increase revs. If I try to increase revs quickly, it doe show a bit of a "stutter" up to 1,000 - 1,200 rpm.
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