James :
Your problems with the UAD sound alot like the de-normalization problem that Pentiums had. But, I haven't heard these issues associated with UAD plugs.
I had the exact same symptoms with some PSP plugs (but quite a while back). Mateuz Wozniak from PSP actually wrote a plug-in to combat the de-normal problem before updating the whole MixPack.
If you do a Google on de-normalization plug-ins you'll find some links to free plug-ins that you insert before the offending UAD (or any other) plug-in.