1500rpm cut out, 2001, S40, 1.9TD, Engine Number D4192T3

Have changed Oil and Filter, Fuel filter and added Diesel treatment but problems persists. When the 90k 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.

Check the wiring to the water temp sensor (r/h side of cyl head as you look at it) make sure that is all ok as these break down over time (known weak spot) also check the loom under the radiator (same weakness here) Sounds like it could be caused by faulty wiring/interrupted signal?

Thanks for this Nickmac, not sure if what I cleaned is what you suggested fixing but your ideas certainly pointed me in the right direction. The car has 90K on it and from when I got it at 80K I wanted to give the engine a cood clean and degrease. The worst mess was all around and underneath the Stop Shutter and Exhaust Gas Recirculation Value. At the service I sprayed degreaser to clean up the area but this made the problem worse. On closer inspection I found an electronic connection just below these units at the back right of the engine block. When I disconnected it I discovered it full of dirt and oil/diesel/degreaser. Since I cleaned it properly the problem hasn't happened and that was 2 days ago, so I'm assuming this was the cause of the problem. I'm not sure what the connection links to but it sounds like the water temp sensor you suggested. Any changes and I'll update but thanks again for your help.

Sounds like the water temp sensor, dodgy readings to the ecu from this can result in a plethora of weird and wonderful problems, none of which are obvious faults. The wiring broke down on my temp sensor and all sorts of things happened; the head lights dimmed, the radiator fan would come on all the time, the temp gauge wobbled around (obvious one!) and the engine revs were hesitant! Damned electronics, more trouble than they are worth half the time!