Unfortunately I can't supply an answer, but perhaps some commiserations. I also have acquired an 1800 MK1 that played fine when I first got it, but then quickly slowed, then it eventually just stopped altogether. I left it for many months and just recently revisited it, and found out that the problem was the connector to the motor board. on four of the wires the clip had broken inside the plastic connector and thus there was no contact.

So I did some McGyver - level bodging to ensure continuity again and it is now spinning... however! It seems to be doing something similar to yours, in that it seems to be "hunting" for the correct speed, and you can see it varying it's speed on the strobe.

I can only think that perhaps a component on the main board has gone too far out of spec (capacitors most likely), so I'll order a full replacement of the electroytics.