Well OP don't take for granted other peoples work on your car. I am referring to comments like...
'Service history shows spark plugs were replaced 4k kms ago so I highly doubt they're cooked already"
Always check it yourself. Especially something so simple that can tell you so much just by pulling plugs, looking at color, checking /adjusting gap, and looking for cracks in the ceramic tip and then making sure they are properly torqued back down. I bring this up because I had a friend with a BMW Z3 and he described his issue and I asked him to check his plugs. He said he had new plugs in it, however after doing as I asked he discovered that 2 of them were loose.
The Ross-Tech cable and its software is not just for error codes. it is a very strong diagnostic tool that will allow you to log what is going on during one of those runs where things are not running right. You can easily monitor and record a run to see if it is a fuel problem, and ignition problem or a boost problem etc.
Without this tool you are relegated to asking people what they think and if they have had issues with similar symptoms. This is not the way to go about it because more than one issue can create similar symptoms. Get the cable and you will be able to determine for yourself just what the issue(s) are causing your problem.