To JTRUSSO:
As soon as you realize you can't hold altitude (or heading), advise ATC. Tell them you're in a wave or that there's a cumulus in front that you don't want to penetrate. You next statement should be "What" you want. Do you want ATC to clear airspace all the way to the ground, do you want a heading change, or do you merely want a block cleared below you. Haven't run into a controller yet who doesn't know what a mountain wave is (particularly if their airspace is in the mountains) or what a towering cumulus is. Taking action early always gives you more options and lets ATC know that you are not intentionally violating clearances.
Terry, CFI
Good advice above...
Remember if you are having trouble, you need to communicate it. Always use the following format for all coms...
Who you are calling, Who you are, where you are at/what is wrong, and what you want to do:
"Jax Center, Experimental 123, experiencing strong updrafts, request (block alt, climb, descent etc)"
"Tower, Experimental 123, holding short 17 at 'Bravo', vfr to the north"
Other good coms advice is never say 'for'... it gets confused with 'four'... build good domestic habits that flow into international ones...
try saying, "Center, Experimental 123, leaving 5,000 climbing 8,000" instead of "Center, experimental 123, leaving 5,000 for 8,000" as some international controllers would get confused and think you are climbing 48,000.... yes I've seen it.... and if I recall correctly a jumbo jet was cleared "descend four four hundred feet" on an approach in the pacific and thought it was "for 400" and flew into the dirt in the 80's....
Though this is a rare problem in the US/Canada train now and you'll be ready when you are a 777 captain