Hoffy, the standard procedure of cleaning the contacts on the back of the lens is worth trying. They are gold plated, so use alcohol or similar on a swab or fine cloth, nothing abrasive. I doubt it's worth repair if repair is what it needs, but maybe. Contact Koh's Camera and see what they think.
EDIT: here's a nice tutorial on disassembly of a 150/3.5 S lens. A PS will be similar.
It shows which is the flash contact on the back of the lens. You could try to trace and look for a bad solder joint inside the lens; if the fault is in the shutter module itself that would be a delicate job if you were motivated to tackle it, but you might succeed. I would only do it for fun. If it came to that it makes more sense to buy another lens and sell for cheap the one you have, to someone who doesn't care about flash sync.