Ok, Listen. On the dashboard you will not get any messages that are engine related. On the cluster text may appear saying you have low coolant, engine oil level, suspenton, tranny, ect... Other, Engine related (for example missfires, ect...) will appear as SES (Service Engine Soon) light, But however, there are priority items, some will trigger SES, some will not. For example when my Thermostat went bad and got stuck in open possition I didn't get SES Unless it realy goes bad. Anyway, sometimes I do a check on the codes and found out it needed to be replaced. So go and get your Codes read, there might be something hiding over there!

haha ok thx will do it this weekend

No rattle !!! But i found this article on ehow : im sure now it s a plugged cat!! http://www.ehow.com/how_6513500_can-...-plugged_.html

My mechanic told me that it could be a failing fuel pump !!! But i dony get cel light . Is that true

Any idea

Can you or your mechanic attatch a fuel pressure gauge to the feed line of the injector rail, and tape the gauge on the windshield and go for a ride. If the pressure gets lower when you loose power, it may be a bad fuel pump or clogged fuel filter.
I don't know if the gaz tank is steel on a X5, but if it is, it may be some rust getting aspirated by the fuel pump, then block the strainer. I had that happened on a 4Runner.

Thx for the tips will do the test this weekend . I called the dealer today and he said :" Some pumps will only fail after warming up, so they appear to work fine after they have "rested" for a while. This is the sign of a dying fuel pump motor" and i have that problem . When the engine becomes cold again it runs fine for 10 min then becomes sluggish and unresponsive and reluctant to downshift

You can attach a pressure gauge to the fuel rail and leave it there long enough to get the motor hot to find out if the pressure is dropping after the system "warms up".

My list of things to check on my own would be: Condition of the air filter, fuel filter, spark, fuel pressure

ECS Tuning

Man, you keep saying you will check this weekend or tomorrow, buy jumping to all theese, you will never solve that problem and car will screw up more. get over with it and check for thsoe problems.

Lol yes you are right!!! Im a medical student and im very busy these days !!! Yesterday i went to my mechanic and i did a tune up: spark plugs air filter fuel filter testing the fuel pump and the cat ( the cat is clean ) changing coolant hoses and overflow reservoir and a new maf sensor !!! And i felt that the x is now a lot more powerful and responsive ! I think the problem was the 11 yrs old maf . ( but i didnt have a cel light ??!!!!!) thx for your tips