Following on from my previous post. I took the car to a Ford /Volvo specialist.

Their opinion was that, if it was a crack in the liner or block, the car would overheat.

As it doesn't overheat, or use much coolant then it could point to a leaky injector
causing the same symptoms as rough starting.

Anybody else out there with an i6?

That’s very good news

If you have an ODB code reader that checks PIDs you can look at the fuel pressure

Verify the pressure with the engine running

Stop the engine and check the pressure it should drop quickly if an injector is leaking

A few things to note

If you bring the engine up to full temperature the fuel pressure should actually rise when you shut down the engine

To make the measurements with the engine off you need to hold the start button for 2 seconds with your foot off the brake to power up the ECU

Good luck

Paul

From my experience, a leaky injector will make the car to start rough when hot, not when cold, as you say. When cold it will start easier.
Check, as Paul said, the fuel pressure to be inside tolerances. It will be best if you are able to log/graph the fuel pressure during engine off, cranking and running.
Meaning, turn the ignition on, without starting the engine, start the logging process and see when the engine is stopped, then when cranking and then when (rough) running the pressure.