Check your polyphony setting for the part. If you're sure the part has more than enough voices, are you running Mac OSX? If so, how many samples have you loaded into MachFive? As a general rule of thumb, you shouldn't load more than about 70% of the total amount of RAM your computer has (+/-10%). For example, if you computer is equipped with 1 GB, don't load more than around 700 MB of samples into MachFive.

Because of Mac OSX's built-in memory management features, there is potentially an unlimited amount of "virtual RAM", but when Mac OSX runs out of real RAM, it starts caching the overflow to disk. This can wreak havoc on MachFive performance. Unfortunately, Mac OSX doesn't provide any means for applications to know - or report to the user - that is has run out of real RAM, so there is no way for MachFive to alert you if Mac OSX is caching MachFive samples to disk. Therefore, if you are loading lots of sample data, you need to keep an eye on how much RAM they use up. There are third-party utilities available that can help you keep tabs on your RAM usage.

Mac OS9 doesn't have this problem, because MachFive gives you an alert that OS9 is running out of RAM.