F355 'Flat Spot' 3000 Rpm

When I slowly open the throttle - car static - engine at full operating temperature the rpm rises smoothly until RPM reaches 3000/3200 rpm then slowly drops to 2000 rpm and then rises again no issue.
When driving if you balance the car on the throttle (light trailing throttle then gently open) when cornering you often feel this dead spot which can make the car a bit twitchy. Its a 94 2,7 - has been there even after a number of dealer services. I assume its a standard problem either a whole in the fuel map or inlet design.

Has anyone experienced this problem or is it me.

I am also changing cam belts at 5 years with no issue - you need to inspect belts for pre-caution - every so often say every year but quite easy with an endoscope.

Any advice on the 'flat spot' would be welcome.