I have a hard time starting the car. When I turn the key, the engine starts but then shuts off. Also when I'm driving, I feel like the engine is struggling to speed up. I even had a time when I was driving on the highway and then got the message "Reduced engine power" and the car speed went down from 100 to 80 km/h. This is the code my mechanic got when the car was hooked up to the machine.

Try cleaning the throttle valve.

Did you check the fuel pump relay ? The relay gets power from a fuse and also from the PCM relay. The computer grounds the relay to turn it on for 3 seconds when you turn the key on, then only after the computer "sees" RPM or oil pressure.

You have to start with the basics. Check for spark then fuel. Spark comes from timing signals from the camshaft and crankshaft position sensors to the computer. Fuel, do you hear the fuel pump start when you turn the key to the on position.

It is most likely one of two issues. Either the Camshaft sensor, or the Crankshaft sensor. Neither is too expensive nor too hard to replace. Try the Cam Sensor first. The 5.2L seems to have these more often. The 2.5L has the same problem, but it's usually the Crank Sensor.

The name of the game is testing ! Scan tool diagnostic's , Doesn't matter if no codes are stored . Viewing engine management sensor data ..
Chevy 2500HD No Start No Spark No Fuel Cranks OK

048 Engine Hard to Start or No Start Scan Data for Starting Problems

I would think the crankshaft position sensor or ignition control module could be the problem , but without testing ,that's just a guess .

Your car should not turn off while driving, After your revived car has been running a few minutes, turn off the engine and see if it will restart on its own, no cables attached. If it starts, you should be okay.-------------------------------------------------------------------------------
KWP2000 Plus ECU REMAP Flasher Tunning

The valve that feeds fuel to engine is not shutting down when you turn off the ignition.

it could possibly be ur idle valve(air)....if there is a hissing sound when u cut the car off??? spray break cleaner toward the engine block and if the shaking stops its def ur idle valve

When the engine is cold, turn the key 2-3 time w/o starting the engine. What you do there is injecting fuel through the starting valve to get started. Normally this is performed by the remaining pressure in the system. Then turn another time and start the engine. If it starts easy then the fuel pump, which has a valve preventing back-flow, is the problem. However if this is the case the car should start quite ok after a stop shorter than say 10-20 minutes.
In case this wont be it, then it could be a problem with the cold start valve, which is a electrically activated valve. The problem might be that this valve does not close completely and leaks extra fuel in the intakes which than would influence the idle. You wont feel it driving, other than good pickup and lower gas mileage.

If you would share with us all that was done , it may help us to diagnose the problem more efficiently.

I spend $2000 at the dealership , doesnt help much.

My first gut feeling is going to be a fuel pressure leak... overnight the fuel pressure could leak down... and cause the engine to starve for fuel durring morning start up.

You can check this by doing the following next time you do your morning start up proceedure.

Turn the key on , but do not attempt start the vehicle yet.
turn the key back off.
turn the key on... until you hear the fuel pump pressure has built again.
turn the key off. and then back on ... and after the pump stops... try to start it.