Olympus 35SP - Shutter trips every 18th frame

Hey all,

Hoping that I'd be able to narrow down possible causes for an issue I am facing.

Picked up a minty 35SP for a great price that was advertised as being 'dusty'. Cleaned it up, re-did the seals

and it all seems fine.

However, when I fire the shutter 18 times, the next time I cock the winder, the shutter trips again. Meaning,

I've just exposed the frame twice. This is an issue that I can repeat (yes, how else would I have been able to

narrow it down to the 18th frame..).

It happens on various aperture and shutter settings.

Anyone have any idea why this is happening? Am assuming that something is not right on the inside, and I'd like

to narrow down the possible areas before I send it off for a repair.

Thanks a lot in advance.

Alvin