Customer complaint flat spot under load, only just purchased car, fault noticed all the time pulls back on constant throttle. Fault not quite as bad cold. 25,000 miles. No other faults.

No fault codes logged
Fuel pressure and flow ok - demand test ok
Plugs replaced when serviced and correct ones
checked cam shaft position sensor wiring ok
O/Test Cam sensor, TPS and map & purge valve all ok
Drained and refilled with fresh fuel
Checked for air leaks

co - 0.02
HC - 1ppm
Lamda 1.004

Any ideas welcome, many thanks Chris