Hello Paul, and welcome.
This sort of behavior has been observed by others (myself included) before. It is due to the automatic temperature compensated tuning, which is trying to keep the synth in tune as the circuits inside warm up.
It should not be this obvious though.
From what I understand, the system takes the internal temperature reading several times a second, and looks at a temperature table in memory to know what appropriate Control Voltage to send to the circuits in order to stay in tune.
But sometimes that table might not be reliable (for example if the room you use the OB6 in has a different temperature behavior than the previous owner's room).
One sure way to fix this, but it might take some time to get back to normal, is to erase that temperature table and start from scratch (pressing MANUAL+3), and then by pressing MANUAL+0 (calling calibration routine) each and every time the synth goes out of tune.
After a short while, the observed behavior seen in your video should be a lot less obvious (but not quite completely gone for the first few minutes that the synth is powered up from cold). After that, the tuning should stay rock solid. Well, as rock solid as analog oscillators go.
Anyway, that worked for me and a few others here who had the same issue.
Hope this helps.
Alain.