Weirdest power loss pattern

Question

I've a CDTI Opel Meriva from 2003. It started having this weird issue 2~ years ago and it hasn't really improved or worsened, but I've taken it to the official Opel service and various mechanics and none had a clue what can be causing this.

So the issue is, the car starts normally and you can run it for a while, everything OK. Now, as time goes on (say 30 minutes, 15 if the car has to go through step roads or you throttle it high) it starts losing power rapidly on the gear you are using. So if you are on the freeway you start losing gear in 5th gear, and in 5 minutes even if you floor the throttle it doesn't rev up at all, 0, it's gone. The curious thing is, you switch to 4th gear and it runs perfectly fine, 100%. But as it goes on with 4th, it starts losing power on it too until it becomes useless. Same happens with the rest of the gears. In the end you have to park the car somewhere and restart it, which seems to solve the issue temporarily.

I'm no car expert at all but it baffles me various experts have seen it and can't identify the problem with such a peculiar pattern, it is driving me crazy (no pun intended). Would someone have any idea of what can be causing this?

Edit: Forgot to add, there is a gradual vibration increase while the engaged gear loses its power. When the gear reaches a point it's useless, the car vibrates a lot while it has that gear engaged.

Jorge.V · Accepted Answer

Answering this for the sake of helping others if they stumble upon here with the same problem.
Finally, in one of those automated checks they plug the car into, compression on one cylinder came as low. The mechanic changed the injector for that cylinder and voila, never had the problem again. I'm no mechanic myself so I don't understand how the behaviour we were experiencing was related to this, but yeah, if this happens to you, check your injectors are working as they should.

Con FUse · Answer

It takes less power to move a car in 4th gear than it does in 5th.
Gradual loss of power sounds like system is clogged - maybe catalytic converter.
Because of the age of the car it should have a closed loop O2 sensor system.  If it were an American car with OBDII, you would see signs like air flow reduction INTO the engine.  Since its an Opel, its unclear.
There is a possibility its a fuel clog.  That could case a lean miss (noise you are hearing).  A fuel pressure gauge could verify this.
To make power, a car must have air/fuel/spark in the right amount.... at the right time.
There is something going on that changes that... since you didn't mention a check engine, presumably that something is not directly monitored.
Start with monitoring the sensors and see if you can figure out which ones are out of spec.  Maybe take the Opel mechanic for a ride and demonstrate what is wrong.

Weirdest power loss pattern

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