1.8 flat spot and poor power

Well, really pleased with my 1.8 Millenium edition. Got it cheap!!
Removed rear prop, amazing difference!!
However, its got a horrible flat spot from tickover, and very, very poor economy and performance. Engine seems reluctant to rev freely.
Valve timing has been checked. New plugs fitted.
I've had 3 K series' in the past, albeit in MGTF's, but the performance is sooo vastly different.
Any ideas please?
Thanks, Russ

Yes,get it on Testbook and do some live data readings - the readings of the Map sensor and Oxygen sensor will be good to check first.I am assuming that the car has had regular servicing,spark plugs/leads/cap and rotor are all up to scratch ?

Thanks...yep, service history is complete. 81,000 miles. Compressions checked-all around 165. have not checked cap and leads yet...no smoke/fumes. One thing though...engine is beautifully quiet at tick over...no rattles etc, but above 3000 rpm it sounds like a bag of nails. Not sure if this could be a resonance/vibration of a heat shield or similar? Its very difficult to tell...What is Testbook?
Thanks, Russell

BTW can anyone shine any light on the rear diff. The car is so different to drive now that its been disconected. Is this normal? also economy seems to have improved from around 20mpg to 30 ish?
Also it is quite noisy above 60mph. Maybe an additive would help?
Thanks