[iemandijenier]

My car seems to be running like it is supposed to, only to start acting up randomly. Usually, this happens after 30 mins+ of driving, or after two short, consecutive trips. Things I've noticed are loss of accelerating power, jerky acceleration (switching between no power at all, to more torque than usual within a second), running low and rough on idle. So I'm leaning towards a bad MAF sensor, but what's puzzling me is how the car can occasionally run smooth for like an hour, only to turn to running badly within seconds, and without a clear reason.  I would appreciate any suggestions

[iemandijenier]

I must add that the car had been sitting still for 6 months prior to this and it had a tuning job done to it by my mechanic

[Endeavor]

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.

[iemandijenier]

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=share

Any suggestions, what parts or sensors I could try checking out?

[iemandijenier]

Also earlier today, i test-drove the i30, to try and isolate the problem. The problem seems to appear after the engine is properly hot. So after 20 minutes of mixed city/highway driving it started holding in and shocking. On cold start, it works flawlessly. Then I continued driving for around an hour, and the problem did not disappear. So at the least, I can say that it is related to the motor's temperature. Once hot the engine loses power, jerks and idles very rough. Once it;s cold, everything works as if there are no problems.

[Endeavor]

Holly smokes, ECU, engine swaps...
I would start by addressing the codes first, and the oil of course. Then see if anything has changed.