01 5 gen throttle stumbling at 4- 6 k rpm?

Gentles all

I have a 01 5 gen with replacement headers and slipon. without cat but with Lambda probes in place.
now recently it has started to stumble a bilt when i aply throttle between 4000 and 6000 rpm
its more pronounced when the engine is cold.

I have not really looked at anything yet, but I hope that somone here will have a clue as to where I should start.


Thanx