Had this on my Tommasini. I used repeated applications of PB Blaster over a period of weeks, from above and below, also with a lot of tapping on the stem with a piece of maple. The wedge/expander was loose from the start, so it wasn't that. I think because I'd been cautioned by [MENTION=84826]randyjawa[/MENTION] I took my time, and never got too crazy in forcing anything. I was also rebuilding other parts of the bike, so I had no reason to rush, and I was hoping to preserve the stem. My patience eventually paid off. I've tried ATF/acetone since then, on a stuck seatpost (no luck on that one), but I have read that it's even better than Kroil, which is apparently better than PB Blaster.

Edit to add: The heat/cold trick didn't do anything for me.