You didn't explain anything. Why would the value of a constant amount of money increasing in proportion to productivity growth lead to "wild fluctuations"?
But that's the same as an admission that rapid deflation (a necessary consequence of the effectively fixed amount of gold) would do so as well. This is true because, logically, the effects of either type of imbalance should be similar, with the only difference being the direction of the final stable point (up in the case of inflation, down in the case of deflation).
Productivity has grown a few percentage points a year, give or take, consistently, for a long, long time. One industry may boom for a decade, but the economy as a whole is relatively stable. I'm still not seeing anything wild.