I have similar problems with my FX45 (2005).
After coldstart the first acceleration is always normal, but after that it loses power significantly (even though the engine is still cold) until the next restart. When I stop the engine and restart it, the first acceleration is normal again, but after that it loses it's power.
I had a new MAF sensor at home as a spare part, so I replaced it. It didn't solve the problem, so I visited a mechanic who tried to read out the fault codes, but there are no fault codes at all and no engine light.
Then we made a test: we measured the engine parameters right after a coldstart (during the 1st acceleration) and we repeated the test after the engine has already lost it's power (2nd acceleration) and we compared the results. The only difference was that the airflow was significantly weaker during test 2. The throttle body is pretty new, so it cannot be dirty. The mechanic has no idea what's this. This car is pretty rare in my country (Hungary), there is no mechanic who is familiar with it.
Did you repair yours?