This morning, as I was driving, I experienced some shuddering when the car slowed down. At idling, the shuddering was more obvious. It was as though the car was trying to find the right gear, or misfired. The initial thought was it might be a coil pack problem, for which I had a spare part as a back up. But as I googled to confirm the root cause of the problem, it looked more likely to be a vacuum line leak as coil pack problem usually causes misfiring at higher rpm (and my engine shuddered at low rpm or idle)
Popping up the bonnet, I inspected the lines and true enough, I found the vacuum check valve broken. Hopefully there is no other leaks, and we will find out more as I bring the car to the workshop tomorrow.