Tape recorder randomly stops playing

slipping belts could even be an underlying cause of your problem

The problem occurs at the beginning of the tape (it might have something to do with the white tape either with the first 5-6 seconds of the recorded tape) so I would agree regarding belts’ issues if it happened during mid-end tape.
Why the problem disappears after 5-6 seconds of recorded tape and just “occasionally” or none reoccurs during the rest of the tape? What are your thoughts?