Have changed Oil and Filter, Fuel filter and added Diesel treatment but problems persists. When the engine is very hot i.e. stuck in traffic, one of two thinks will happen, either the engine will cut out after a couple of minutes of being left idle or will refuse to rev above 1500 rpm when I try and drive off in first gear. The coolant level is fine and unchanging and the fan is working. This later version of the problem is really dangerous as the car will cut out really quickly if you try and engage first gear and accelerate away from a junction or traffic lights. What I normally have to do is rev the engine a few times until I clear the 1500rpm limit at which stage the car drives off fine. For this to happen the engine sounds like it has something stuck that eventually frees up and allows normal operation. If the engine cuts out it always starts fine. Once the problem (blockage?) clears the car drives perfect through all the gears. From a cold start the problem doesn't present itself for about 20 minutes or so i.e. when the engine has got to a high temperature (I think).

Any ideas or help on what could be the problem would be much appreciated.