I had a similar problem with Microsoft's softsynth on my laptop.
In my case, the problem was caused by the CPU power management. Most laptops these days change the CPU speed based on the current load.
If you are running Windows XP, go to the Power Options control panel and set it to "Always On" (CPU will be at 100% all the time) or "Desktop" (CPU will be at 100% when on AC power and scale down when on battery power). Which power profile you select affects the dynamic CPU speed and settings for these are not exposed. After choosing a power profile, you can change the rest of the settings (time before display off, sleep, etc.) to your liking.
In Windows Vista, these settings are exposed. Go to the Power Options control panel, click on "Change plan settings" for the plan you are using and then "Change advanced power settings." Look in here for the CPU speed settings (I can't give the exact location because I'm not on my laptop) and make sure the minimum and maximum are both set to 100%.