Jerky Acceleration 406LX 2 litre petrol

[Phil]

The engine is fine when cold or when warming up however, when the engine gets up to normal running temperature it starts to jerk and stutter on acceleration with wide throttle opening at low revs. When the engine revs come up the stuttering ceases. There is no smoke from the exhaust or any backfiring so it seems like fuel starvation. I have replaced the MAP sensor, the Lambda/oxygen sensor, the idle control stepper motor and the spark plugs but the problem remains.
Any advice would be much appreciated.

Yesterday I removed the fuel pressure regulator and vacuum tested it to make sure there were no splits in the diaphragm.
The two neoprene rubber "O" rings looked in good shape so I re-fitted the regulator.
Since doing this the problem still remains but nowhere near as bad.

[Phil]

The engine coolant temperature gauge is working correctly and giving normal
readings so I had no reason to suspect that the cooling system could be causing
the jerky acceleration problem. However, when I went to top up the coolant in the
expansion tank I needed 2 litres of water / antifreeze mixture to bring the level up to maximum.
Now the jerky acceleration problem has gone.
Strangely I can't find any water leaks so I'm wondering if there was air trapped in the cooling
system from when I last drained and replaced the coolant.
I assume the lack of coolant was causing something to overheat even though the temperature
gauge was reading normally.