It just seems odd to me that it happens at frame 5 all the time.
Seems if the teeth mating were slipping it would be more random but I'm just guessing.
You can check the "feet" also...the little things on the body at the bottom.
These need to be within spec to hold the back firmly to the body so nothing can slip.
I know there is a jig that the 'blad repair guys use to make those perfect because the can bend sometimes.
As far as what would be cheaper...it's really a crapshoot unless you can get a good warranty but even then, the back could need servicing after a year anyway.
I have around 5-6 backs (all bought used unseen) of all types from the "peep hole" 12's up to a newer A12 and I've been lucky.
No service to any of them but I don't run a lot of film like a commercial operation would've back in the glory days.