Hijet stalls while cold!

My Daihatsu Hijet 2005 requires a ten-minute static warm up time.  Otherwise, engine will stall when driving and taking foot off the throttle.  Once warmed the engine is fine.

I’ve noticed that when starting from cold the revs aren’t as high and smooth as before.  Engine runs fairly ragged until warmed up.

My mechanic has checked the air filter, changed spark plugs and coils. Also manifold, Lambda & engine temp sensors.  Any ideas?