DESPERATE-very bad engine problems

[codered]

Hello,
I want to start by THANK YOU to anyone who will reply,I am really desperate because of my engine problems with my X5 (E53,2000,4.4i/286HP,163.000km)
It started 3 months ago,the yellow light (engine service) flashing while the car was running on high RPM's...it flashed couple of times,then it stayed ON.
Now the car has lost significant amount of power,it accelerates slower and is not running more than 160-180km/hour.Even at this speed if it's being "pushed" the engine starts shaking badly and it needs an engine stop,then it will run again.Anywhere near 4000rpm's the engine shaking starts again.
Before it starts shaking the yellow light is flashing again(when the acceleration is being pushed towards the end)
I went to my mechanic,he reset the sensor to see what is happening.After around 100km it went back on again.
The software reading from the tester gives different results depending on the actual software being used
On one tester it said the lambda sensors after the catalytic converters need changing.I changed those,still no result.
On a different tester it said the camshaft sensor and the acceleration sensor need changing(haven't changed them yet) It also says "MISFIRE on Cylinder 1,6,8" I've also changed the ignition sparks and engine oil.
On another tester it said the thermostat (hopefully it's the same in english as it is in romanian) and the engine fun needs changing,but this seems curious to me,as the engine temperature is normal .Also,the tester should not be able to tell if these are damaged?

WHAT SHOULD I CHANGE ? I don't wanna change something that it works,it already cost me a fortune fixing other stuff.

Please,help...very much appreciated.