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.