Mk 2 Engine Hesitation at around 2,500 rpm. Get's worse as engine get warmer

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: 2000 Mk 2 1840cc
  2. I’m based near: _Amersham
  3. I’m looking for technical help or recommendations on: Solving engine hesitation/stutter at 2,500-3,000 rpm when accelerating. Symptom gets much worse as the engine heats up. For the first 5 mins of any drive it runs fine with no symptoms, but after that its starts hesitating, and then after about 15 mins it almost become undrivable, and you have to limp along at around 2,000rpm, minimise acceleration, and anything above 3,000rpm causes sever stuttering.

I’ve replaced the coil pack, ignition leads, spark plugs, and most recently fitted a new O2 sensor to the exhaust manifold. None of these changes, including disconnecting the front 02 sensor seem to make any difference. I’m not seeing any error codes (using the LED on the diagnostic port).

Any suggestions of what to try next ?

Thanks