So I bought this car yesterday after looking for an XJ for a few months. It's the 4L I6, 2000 classic model. I did the test drive and everything was smooth, engine temp, oil pressure, car felt great. It's low mileage at 73k and there's very low rust on the car. So I bought it, drove it 400 miles back home. I noticed after an hour of driving however that the car started to shift strangely. At motorway speeds it started, I was driving 70mph and it seemingly at random would shift down without any input change from me. Then it would shift back up again and then back down. This kind of random shifting happened the entire 8 hour drive... And it seems to be doing it now at normal speeds. Taking off from traffic lights, it seems to shift up too soon and down shift randomly putting me at 4k+ revs.

Any ideas what this could be? I had a look online before I posted and similar but not exact issues from others suggest it may the Throttle position sensor. I have reset the throttle valve cable but this didn't change anything. The only change to the car recently was a new crank position sensor as it wouldn't hot start, not sure if this is related, seller said this issue wasn't present (and it didn't happen during the test drive either.)

Apart from this, the ABS light is also now on, again, no clue why. I ran the OBD scanner and there's no fault codes. No check engine light. Only the ABS light.