OK, I disabled some UAD plug instances that I didn't absolutely need (consolidated them into a bus), so now things are OK. I thought UAD DSP usage had no effect on CPU hits, but since my UAD usage was getting close to max, maybe it does at that point. Still, I wasn't having any problems until I started using Melodyne, and that was before my UAD was getting close to being maxed out.

Perhaps you are over taxing your CPU with a small buffer.  Have you tried raising your audio buffer (particularly when using Melodyne)?

+1
 
Melodyne (and ARA in general) take advantage of looking at a wider window of time when editing data (not simply the traditional "time now" approach). Maxing out a 6-core I7 is quite a feat, actually, so your buffers may not be giving Melodyne enough data to play with smoothly.
 
Try adjusting your ASIO latency up (Edit->Preferences-Audio->Driver Settings) a step or two at a time until your buffer samples get closer to the "1024 samples" that Melodyne recommends and see if the CPU/crackling issues gets better. The I/O buffers will probably be best at ~512 (Edit->Preferences-Audio->Sync and Caching (Advanced Mode)).
 

I haven't had any problems yet but while I use it a lot in my projects I don't often have more than two or three instances playing simultaneously. Maybe there's a difference between the monophonic and polyphonic ones too? Kinda surprised really. Melodyne isn't that new and always ran fine on my systems, down to a Pentium single core I think.

Oh yeah, sorry, I should have mentioned that I had my buffer set at 1024 samples. In fact, I had that already set even before using Melodyne. And what's strange is that upon my initial use of it, it still alerted me about increasing the buffer setting to 1024, as if I hadn't already. ???