Kind of curious - it happens somewhat frequently in weather in Florida that I will hit a cloud street of thermals that makes it impossible to hold altitude or descend. I got blown up 1000 feet in about 20 seconds last week (and back down on the other side). Not turbulent, but, well, Florida can do that.
Usually, it is when I have already asked for a steer around it, but once or twice ATC hasn't been cool with reality. What's the best way to give a heads up that altitude is not entirely something I am entirely controlling?
Personally I'd rather have you just ask for a block but at least let the controller know what's up.