Fabia 1.2 (2010) 12v loses power during warm up

During cold weather when starting from cold, the car loses power during the first couple of miles.  When driven slowly, it needs revved to maintain progress or a very low gear.  On a dual carriageway max speed is 45mph under full throttle until the problem passes after 4 or 5 miles.

Local independent VW Audi specialist cleaned the throttle body and readapted but the problem persists.  On further visits to the garage they are unable to trace the problem, partly because it is transient and only occurs from a cold start.

It is a Fabia 1.2 12 v DOHC 3 cyl.  First registered July 2010.  Only 40,000 miles.  Good service history, Recent coil packs and plugs. We have had the car a couple of years and this only became apparent in the autumn.

Excellent car apart from this problem.  Would be really happy if this can be fixed.