[NotAMechanic]

Hello everyone!

I have an issue with my 2009 i30 1.6 CRDi. I've searched everywhere and found some similar posts but none of the solutions worked.
When the temperature is below ~15°C, the car always starts flawlessly. However, in warm weather, the RPM keeps dropping and increasing for 5-10 seconds (closer to 5). This only happens if the cars sits for a couple of hours. If the engine runs for a minute or so, the next start is perfect again.
The fuel filter is new and so is the battery. I changed crankshaft sensor, camshaft sensor and mass air flow sensor. I removed injectors and had them diagnosed. They are not perfect but should still be fine. I also checked glow plugs and replaced a defective one without any improvement.
Those were all the clues I found on this and other forums.

This is my first attempt to repair anything in any car so I'd greatly appreciate the wisdom of this forum.