Hi! This awkward situation has happend to me too a few times. I think this is RIO (road induced oscillation) and occur at a wavy or uneven road surface and lower speed......in my case 40 mph and lower. Cause.. probably too stiff suspension which doesent react quickly enough to smooth out the uneveness on the road.
Yes, this was really an awkward situation. It was like having one or few very oval wheels bumping the car up and down. I too checked the wheels at home but could not find anything wrong with them. Bearings front and aft was done and tensioned as per Clarkes-Garage a year earlier so I donīt think this is the issue.
Remedy? Well I pulled aside and zeroed speed and gear then started up again with forceful accelerating to road speed. No problem now.
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Lapponia 1984 944 na