What you are experiencing is a performance issue with how the audio buffer is handling audio at low latency settings. I would first download and install the latest driver update for the Presonus interface.

You could also see if there is a RAM preload setting inside the VST instruments that you are using. 2GB should be enough actual RAM. It would be good if you could raise the RAM preload settings inside the VST instruments if that is an option available. Not all VST instruments have this option.

Hope this helps.