It's plausible that the record is slipping due to insufficient friction between it and the platter.  Especially given the heavy, crude tonearm.

Just wondering - if you don't know much about record players, but are interested in repairing them - I don't get it.  You just want to learn about these players?  They were not made for music reproduction, they were not meant for that.

I would strongly advise you to quit playing your records on that turntable; you are almost certainly doing

permanent damage to them every time you play them.

Used turntables are not a good idea, because they usually come with problems; expecially cheap ones.

It is foolish to save some money on the turntable and destroy all of your records.

A turntable that old is almost certain to have wornout bearings.

You can get an Audio-Technica LP60 from Needle Doctor for $149.

BTW- that link you posted is infected with a virus according to my anti-virus software; you should remove it.