I'm using DOSBox 0.63 on MacOS X 10.3.6
I have a Dual 1.33ghz G4 tower with a Radeon 9800 Pro and 896 megs of ram. I'm not trying to play any spectacular game (ie. the game runs fine on a 286).

I initially ran DOSBox with the default settings (no config file and such) and everything was running great. Sound emulation was perfect, framerate was great, and I was using about 50% of my cpu power.

Then I changed the config file to do fullscreen. And upon exiting from DOSBox, my computer hardlocked. I rebooted, and then DOSBox ran VERY slow. I tried reverting to default settings, but that was no help. I also tried to tweak the settings, but that didnt' help either. I think something's pretty badly messed up here.