This is definitely a memory-related issue. I assume your Windows 7 is 64 bit, so you should have up to 4GB for TG2, in practice about 3.5GB. The thing is the more threads you use, the more memory is needed for render caches (the cache size is split among each thread). I use up to 8 render threads with pretty complex scenes and it works well, but you need to keep the cache size down. 100MB per thread is usually fine, but in some extreme cases you need to reduce to 50MB per thread. It will warn you about this as a potential performance issue, and sometimes it *will* make things a bit slower, but you'll have a better chance of succeeding. The one thing I do see a lot is people assuming that setting a larger render cache is a good idea, for performance or whatever, but this is definitely *not* a good idea, especially in complex scenes.
In the future when TG2 has a 64 bit binary, memory issues like this should be a thing of the past on high-memory systems.
- Oshyan