Stuttering playback even with small sound font + 6GB RAM

I'm not trying for high quality playback with my computer running an Intel Core 2 Duo 2.2 GHz processor, Windows 8 (64-bit), 6 GB RAM, Realtek sound card with latest Realtek driver, and MuseScore 1.2. But a few seconds into playback, MuseScore starts stuttering and lagging. I've tried using two soundfonts smaller than the default: yamaha xg sound set re-map.sf2 from sf2midi.com (3.67 Mb) and Scc1t2.sf2 from soundfonts.homemusician.net. (3.13 Mb). I made sure all the special effects available from my Realtek driver are turned off. I've tried different Portaudio settings in MuseScore I/O preferences. (It seems that I need to keep "Use internal synthesizer" and "Portaudio" checked, and "MME" selected, or I don't get any playback at all. I've restarted MuseScore after every setting change.) No improvement with anything I've tried! What could be the matter?? Thanks for any help.