300 E with no acceleration - will not rev past 4500 RPM

I have a problem with a 1992 Mercedes 300E. The car has done 43000 miles, and runs very smoothly, but will not achieve more than 4500RPM. Even with it in PARK, I can only manage to get just over 5000 RPM. When you bury the throttle peddle ( with the car in PARK), it quickly revs to about 2500RPM, then the revs take a little longer to 3500RPM, then really struggle to get up to the 5000RPM. When Driving, kicking it down often slows the car down, as you are calling for more revs than the engine will easily deliver. Also when you set off, with the throttle half pressed, it changes up with a jerk, obviously as the gearbox is expecting much more power at that throttle position.
I have tried and tested the following to try to correct it.
1. Checked the air filter, which looked clean, and tried revving the car with the air filter removed. No noticeable change
2. Replaced the fuel filter, with no noticeable change
3. Changed the spark plugs. Managed to get from 4500RPM to about 4750RPM
4. Checked the cylinder compression - all OK.
5. Checked the timing (dealers checked on computer, advances OK to about 40 degrees BTDC)
6. Checked timing chain to make sure it had not jumped a tooth or stretched - Chain installed correctly
7. Check fuel pump delivery flows and pressures with the engine OFF, and are as follows. Open ended flow (no restriction), gives 2400 litters per minute. 5 bar pressure gives a flow of 1860 liters per minute, 7 bar pressure gives a flow of 1620 liters per minute, 9 bar pressure gives a flow of 1140 liters per minute, 11 bar pressure gives a flow of 600 liters per minute. These according to MB dealer are all OK.
8. Tested the fuel supply pressure with the engine running to make sure the fuel regulator was working OK. The delivery pressure held at 5.5 bar regardless of REVS. Suggests it is OK.
9. Tested the air flow sensor, and found the resistance to be none linear. Around the middle of the range, it did not really go up, but jumped around, then at the bottom end of the range went linear again. Tried a resistance box onto the sensor plug, to emulate the sensor output resistance, this made no noticeable change to the engine revs. When the plug was removed from the resistance box, or from the sensor when the engine was running, the engine revs changes for a fraction of a second, then recovered to normal.
10. Lastly, the only thing I could then think of was the catalyst was blocked, cut this out, and welded new pipe in place, and still no change (well very little about an extra 200-400 RPM). Even tried revving with no exhaust at all, with no change.
I have now run out of Ideas, has any one got any clues????
This should be quite straight forward for the dealers to solve as it is NOT an intermittent problem it is like this all the time, but the dealers solution to all the problems is throw enough money at it and they will solve the problem