[LAU] Crackling in ZynAddSubFX

I'm very new to Linux audio recording, so I apologize in advance if I'm missing something obvious. I have a brand new computer with an M-Audio Delta 44 audio card running the latest version of Ubuntu Studio 8.04. I've been experimenting with software synthesizers using a USB MIDI keyboard, and I've managed to get QSynth working very well and have had some success recording that. I thought I'd try ZynAddSubFX since it sounds amazing. The problem I'm having is that the sound crackles (that's the best word I can think of to describe it) when I play it. The very odd thing I've discovered though is that if I record ZynAddSubFX in Ardour, I don't hear any crackling during playback of the recording. It's only as I'm listening to ZynAdd in real-time as I play it. I've tried using different speakers and headphones, but it makes no difference. I suppose I could just record it and try to ignore the crackling as I play, but it would obviously be ideal to have no crackling at all. I've tried tweaking both the ZynAdd settings and JACK settings (although my guess is that JACK settings are fine, since I haven't had any issues with recording live instruments or recording QSynth) but to no avail. I have the sample rate in ZynAdd matching the one in JACK (48000Hz) and have tried different buffer sizes in ZynAdd but haven't noticed a difference. I've also noticed that it usually doesn't crackle when I'm using the default instrument in ZynAdd, only once I switch to a different one -- then it seems pretty continuous no matter which instrument I switch to after that. Does anyone have any ideas as to what could be causing this?