I think poor timekeeping is related to the clutch slipping. There isn't really a fix for this - although I've read some guys saying it can be fixed by roughing up the clutch plates. Accepted wisdom is that keeping the chrono running theoretically reduces wear. I have quite a few 6139/8 chronos and always wear them with the chrono running - they don't have a separate running seconds hand.
I happen to have a 6139 7080 too.
Via the interweb from a land downunder