2.2 DI engine on Almera Tino problem?

Can anyone help with following fault?

Engine starts perfectly.

Problems occur when running with engine cool.

Between 1500 - 2000 rpm engine power fails. Revs drop off below 1500 rpm when you can attempted to increase throttle again.

When this occurs engine noise increases, the car rpm will not increase and white smoke produced.

Once the engine is warmed it runs smoothly sometimes feel a little rough.

Once cruising along no problems at all.

Been in touch with Nissan Dealership and don't like what I'm being told.

Does anyone have any ideas?

Thanks

Just to add the car is a 51 (2002) registered with 95k on clock.

Thanks again.

The cylinder head gasket is on the way out?

Check the oil. I f it looks like cream then water is getting into the oil confirming my suspicion above.

Thanks for that Fidei

The oil is fine. I change this regularly and with filter. This problem has existed since about June / July time.

I've done all the basics of fuel filter and air filter renewal.

I am thinking the problem stems from ECM and its sensors or timing chain or injector pump set up.

I am gradually eliminating each sensor electronically with continuity checks, resistance checks and signal voltage and pulse checks.

Is there an electro mechanically minded saviour out there. I'm OK with the electronics.

Thanks again.