Back wheel moving to left side Can't seem to find something similar so I am going to ask a question.

I recently bought a vintage(mid or late 80s) steel road bike and the back wheel seems to be moving off center and towards the left side of the rear triangle bottom part. The wheel is a tiny bit untrue but nothing out of the ordinary so I don't think that this is the issue. If I take the wheel off and put it back on again correctly it just moves a little bit to the left side after 10-20km. Any ideas on why this might be happening? First time I noticed it was because the tyre started rubbing on the frame and I don't want this to happen anymore.

Cheers,
Ivan
Thanks a lot for the comments, they did help a little at the time but did not solve the issue fully.
At the end I found there were a few issues with the rear of that bike.
The freewheel was wobbling a bit and had to be changed. The bearings of the hub had to be changed and the axle was bent so that was changed, too. Finally, the dropout was a bit bent, too on one side. After changing/correcting all of these the issue did disappear. But it was costlier than I had hoped  Unfortunately I cannot be sure which of these was the main issue.