How to prevent sound being sporadically choppy when playing Amiga games on WinUAE?

I find that when playing Amiga games in WinUAE, the sound goes choppy for no apparent reason. Sometimes this happens right away when starting emulation, other times sound is fine for a while then starts to go choppy at some apparently random point. Is there anything that can be done to prevent this from happening?

My system:

• Windows XP SP 3.
• AMD Athlon 64 X2 Dual Core Processor 4400+, 1.98GB RAM, NVIDIA GeForce 6100 nForce 405.

Some more details:

• I am using version 2.3.2.0, but have also tried 2.4.1 and had the same result.
• I've changed settings, such as sound buffer settings - but this doesn't appear to help.
• The problem appears to happen with any game - not just a specific game.
• The SND: value at the bottom of the WinUAE window often goes negative and is sometimes greyed-out. What does this mean?
• I don't have any sound issues when using other non-WinUAE software on the same PC.