Yep, I'm familiar with the normal stretch- give and take- of cable type clutches. This however, seems a bit excessive. Like you, I start out with perhaps a 1/8th inch of play but it quickly opens to 3/8 and more in very little time.......maybe ten to fifteen minutes of riding time. Haven't ever ridden a bike that exhibited such profound clutch lever play changes in such a short period of time. And this isn't just a matter of a new clutch cable needing time to stretch out to maximum length.