Flat spot and reluctance to pick up speed.

I have a C3 2006 petrol, manual drive. My problem is this; at around 1800 RPM there is a slight flat spot on getting going. It is worse if I am on an incline or a hill. Travelling in traffic at these revs. ( in top gear) can be annoying and I some times have to drop it into 3rd.

The Citroen garage says there is no fault as nothing shows on their computer, however the technician (mechanic) when I eventually got him to drive the car with me, admitted that there was a fault, but did not know how to fix it. His boss said likewise. so much for computers! I am an ex mechanic of the old plugs and points era and can fix most things but this has me beat. I have done all the obvious, new plugs, cleaned the crank sensor, checked for dubious earths on cables, etc. Can anyone throw any light on this problem please? By the way, it is worse when the car is cold after just starting up.

Thanking you in anticipation, Pete.