Suzuki DF 9.9A Engine stalls whilst trolling.

Bought from new, engine has done about 45 hrs but has an irritating stalling problem whilst in gear at tick over speed.. No problems at any other speed.
When trolling, after a few moments, the engine has an occasional hesitation / misfire then just cuts out and there is a noticeable smell of fuel. She then starts up perfectly and is fine until we drop back to trolling again. The idle speed is set as per service manual,approx 950rpm and I have tried it higher but the problem persists.

Carb has been dismantled and thoroughly ultrasonically cleaned many times including the idle jet and passage ways and checked the needle valve and float level but the stalling at idle speed is always there.

What gets me, at home in the water tank, It will idle in gear for ages, perfectly with no hesitation or miss fire at all. Maybe there's a higher load on the engine when it's trolling?

I'm wondering if it might be an ignition problem because of the noticeable smell of unburnt fuel when it stalls but the plugs look good at 40 hrs, the gaps are correct and valve clearances are in spec.

Anyone else suffered this at all or any ideas please?

Update.

Stalling problem now fixed. I reluctantly fitted new spark plugs (at 40 odd hours the existing plug electrodes looked newer than the brand new NGK replacements) and a new fuel filter (the existing one was as clean as a whistle) but I think the real problem was that when the Suzuki dealer did the last service, (to comply with the warranty) they had overfilled the engine with oil. The dipstick was showing a good 1/4 inch overfilled. I drained off a cupful to the correct level and think maybe the high oil level created an excessive mist in the crankcase which found its way through the breather hose into the carburettor and at trolling speed upset the fuel ratio and stalled the engine. Maybe the reason I could not get the engine to stall at home in the water tank was because the engine was never run at high enough revs long enough to make sufficient oil mist as it would have done out at sea on the way to the trolling area. So what a difference, the engine runs better than ever now at all speeds