Isambard, if the surging felt is while cruising around 40mph-ish it could be transmission related. What happens is the torque converter lockup clutch circuit acts up and doesn't fully engage the lockup clutch, but pulses it momentarily repeatedly about once every second or so. The fix for this is to replace the mechatronics unit (valve body).

A quick way to confirm it would be to get it doing it, then while maintaining the same speed shift into sport mode. It should stop doing it. If memory serves it may do it in sport mode at a higher speed, like 55-60 mph?

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