in a 'blue moon' after I drive approximately 100 km (62 mi) or for close to an hour - my transmission shifts up and down very 'rough' x2-3x @ speeds near 70 km (43mph)

it doesn't always happen . I have driven many trips over several weeks without it happening at all.

car has approximately 219,000km (136 mi)

any ideas of the cause ?

Could be a number of things. I suggest having the car scanned for any history or current codes. Yes, the trans can post a code, but not show the light.

Welcome to the Forum.
I agree, having a history & current code scan may point to the problem area.
A more advanced scanner is required, I had mine initially read by a Snap On scanner.
4t65E can develop shifting irregularities, and we will know more once the codes are read.
Please back the code results and can you tell us if the transmission fluid & filter was serviced recently?
Is your trans dipstick fluid on the full line after it'* hot/driving for 20 minutes?

Do a pan drop transmission filter change and put a shift kit in the accumulator while you have the pan off. As your trans is wearing with age it doesn't shift as quickly as it used to, and the PCS turns up the pressure to compensate. Or, it could be the PCS starting to fail. This can result in harsh shifts. Putting in a shift kit will make if shift more crisply and perhaps correct the problem. You are probably headed toward a rebuild, and an oil/filter change along with a shift kit might delay the inevitable.