VW Passat Wagon 2.0 TDI 125kw.
4 years old
2010 June Built.
55000Kms

Not sure if this is shuddering. But took the car out of the Garage on the way to Service the car. Accelerated and there wasn't enough acceleration. felt like not all cylinders were firing. bit of shuddering. drove for 50 meters and started getting better. I pulled over to a parking bay and then started again. All was fine. No error messages or lights. Went straight to the Dealership as I was on my way to service and reported the issue.
Not the first time that I noticed an issue. About a month ago, driving back from holidays, 100Kms on the highway. There was an accident and we were stuck for 30 mins behind. Car was on Idle all the while. When the traffic started moving, there wasn't enough acceleration for 20 meters and then it picked up. Again no lights and no messages.

The Dealership mentioned they ran all tests and could not find the fault. I am a bit worried. Should I be? Any ideas what could be the fault?