If manual trans , this as described seems like bad fuel. Run highest octane fuel + some additives. Next to look is fuel filter. If you refuel in one gas station ... change the location.
If auto, then check/ change the trans fluid.
I am no expert , just suggestion based on my experience. Let us know if this or what solved your issue.
Thanks first of all for your reply
So I ran my OBD meter and it indicated two relevant engine codes:
P0012 Camshaft Position A - Timing Over-Retarded (Bank 1)
P0301 Cylinder 1 misfire detected (severity 2/3)
Info that could be relevant
The car had a new ECU 3 years ago
The car got a new clutch and gearbox 1.5 years ago
Then in march it got a whole new engine (2nd hand, ran 37k, so camshaft should be relatively unworn, but maybe ECU just rejects it)
Then I remembered that the engine oil hadn't been swapped, due to it standing still for 6 months at the mechanic's) since the engine swap and decided to replace the oil and filter today
now I noticed the engine seems to be leaking oil. I hadn't seen any puddles, but the bottom is covered by oil. Tomorrow once I have proper jackstands, I'll try to do a wipedown and oil swap. And see if anything improves or continues leaking.
http://youtube.com/shorts/lplz5BgJtt0?feature=shareAny suggestions, what parts or sensors I could try checking out?