I didn't mention gust direction. I have been assuming it was a vertical gust lifting the tail, but I have no direct evidence for that.

In a nutshell, the comments offered seem to boil down to the following:
a) check flight habits for unintended movements
b) pull stick back to reduce tail-up pitching and roll into the thermal so indicated
c) ignore it
d) haven't seen/experienced it
e) moving CG forward would exacerbate the rocking effect of the tail-up pitching

I think about the rocking of a sailboat-- the retired coach says to move everyone toward the keel when dealing with waves so that the boat can rock fore and aft more easily (and thus lose less speed by bobbing a little more with the waves instead of slamming hard through the waves). From there I extrapolate that a wider weight spread toward the edges (fore and aft) can increase inertia and slow down oscillation about a central point.

Seems the thing to do is to add ballast on the next flight, hope for strong thermal activity, and report back to the thread. More weight will inevitably move the CG forward; can't move myself backward or I wouldn't be able to reach the controls!

Thanks,
Chris