Hi Andy,
We're in the middle of rendering animation of heavy scene with animating trees ... and we also had such issues.
We gave a try with different way of animated trees (native GrowFX, Instanced mesh with PointCache native to growFX, Instanced trees with 3dsmax PointCache) - final solution was making them as animated proxy (even if they were bigger than Mesh+PointCache) and switch to new 1.6 Corona Build.
All slaves (via BackBurner) was Pursing forever, or they started to render 3,4 frames and then next one 10x longer ... solution was ... stupid ... make windows PageFile as big as possible ( I did 40GB here (if you have spare disk space))
Now it renders ... and I just need to restart some slaves from time to time if they have pant.
PEgaz