Hesitation and low idle; threatened cut off at start and stop '04 Outback

[LaBa]

Experiencing "coughing" on start up, and stops as if congested somewhere during the middle of an outing of several short start-and-stops, though it has done it once at the initial start up. Revving the engine with foot on the brake overrides it, and engine dieing is not usually a threat once I get going. 158,000 m and recent spark plug replacement. My Sub-mechanic is baffled. I'm leaving it again to see if it will misbehave for him. Any ideas of where to steer him?