BTW, it seems to happen after the engine has sat for awhile
(months) and during cold weather starts....

Does it have hydraulic "lifters" like a US car? I had a couple of the
ones in a V8 Ford bleed down so far that the push rods actually came out
on two exhaust valves - this after a freeway run from LA to SD. When I
started the motor the next AM these cylinders filled and fired but then
the intake pushrods bent trying to open the valve against pressure. Went
back to the 20w40 Ford recommended (vs 5w30 Mobile one) and never
happened again. Of course it hadn't happened in 20,000 mi on the Mobile
1 either so prolly a coincidental set of freak conditions. Anyway, one
more thing to look at ....