[linux-audio-user] ZynAddSubFX CPU usage

Has anyone noticed strange CPU-usage patterns when running ZynAddSubFX? I'm running DeMuDi 1.2.1-rc2 and Zyn (compile date May 18) from DeMuDi apt-get, and Jack is using 2 periods of length 1024 (it's an old machine, hence the high latency). Things work ok with other synths. Sometimes the CPU usage reported by QJackCtl is very low - less than 2% - while running Zyn (and while it's producing sounds, even on more than one channel). Other times CPU usage rises to 20% and more (even if I switch all notes off), and I start to see xruns and hear lots of glitches. I *think* that changing patches sometimes triggers the change from one mode to the other, but I can't reproduce it. I've tried experimenting with turning on and off eg the reverb module, the Add, Sub, and Pad synth modules, turning on and off notes and channels, but I can't see a pattern to the CPU usage. Any ideas anyone? I'd love to solve this problem, cos I want to use Zyn for a gig on Sunday!