TRY DIFFERENT GRAPHICS MODE SETUP AND COMPATIBILITY SETINGS
Launch the the "Graphic mode setup" utility and try different graphics mode settings before launching the game again. DOSBox games can work differently when using different modes on different systems - we generally pick the setting which we think is optimal, but this may vary from PC to PC.
To access the utility on Windows XP, Vista or 7 go here: Start Menu > All Programs > GOG.com > *gametitle*.
To access it on Windows 8 or 8.1, go to the game's directory (default: C:GOG Games*gametitle*), right-click the "Launch Graphic Mode Setup" shortcut, and select "Run as administrator".
If that doesn't help, please right-click the game's shortcut, go into Properties, click the Compatibility tab and try different compatibility modes as well. As with the settings above, various compatbility modes work differently from system to system, so a trial-and-error approach is necessary in some cases.
EXPERIMENT WITH DOSBOX'S SPEED IN-GAME
Try manually tuning DOSBox's cycle setting, to see if there is a setting that works best for your system.
To do this, press Ctrl+F11 to decrease, and Ctrl+F12 to increase the setting.
MODIFY THE GAME'S .CONF FILE AND TRY DIFFERENT [mixer] SETTINGS
Specifically, try various "prebuffer" and "blocksize" settings.
If you don't feel comfortable making such changes, please contact Support - we will help you out.