First culprit is a dirty Idle Air Valve - the shiny alloy can underneath the inlet manifold. It needs removing and cleaning with carb cleaner quite regularly otherwise it starts to gum up with fine soot, causing the sliding valve to stick.
You can often sometimes nudge it by blipping the throttle.
Next is the Throttle Body, the small brass nipple on top of the inlet manifold, and the flame trap pipework.
Check for leaks in the convoluted air intake hose, loose hose clips, air filter.
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