I have had a similar problem before, but not (I don't think) connected with Railclone or Forest Pack. I noticed this problem in the past when I was rendering very large scenes using distributed rendering with many additional buckets. I contacted Chaos group about it and it was suggested that it was a problem with the render management within vray. The solution (which I use regularly now in order to get a more stable rendering experience) was to use a script to set the number of cores used for rendering to lower than the total number available on the local machine. I use a workstation with 2 CPUs that are hyperthreaded, so there are 24 virtual cores. You can't (aparently) only disable 1 virtual core (from the rendering engine), you have to disable 1 real core, which is therefore two virtual cores.
The script is very simple, just one line of code:
Where 22 is the number of virtual cores you have on your local machine, minus 2 (if your machine is hyperthreaded). This script can be run at render time, through the render dialogue. You can make this line of code into a run-able max script by simply pasting it into a notepad document, and saving it with the suffix .ms
This will set the current session of max to only use the number of cores set to render with, which will leave 2 cores available for render management.
Restarting max will reset this setting, or running the script with the full number of cores will also reset it.
There is also another possible problem that you might be facing: throttling due to memory restriction! It would be a memory setting within vray that could cause this. If you have a very large amount of assets, particularly v-ray proxies, then the "dynamic memory limit" from the render "settings" tab, and the "System" seciton, will kick in. By default it is set to 4000MB. If you are using a very large amount of assets, this limit maxes out and there is a constant loading and unloading of assets in order to continue to render. This (I believe) can mean that the CPU doesn't have much to do while this loading and unloading is happening. It can be solved by increasing the memory limit so that there is more available, or, if you have a very large amount of RAM, you can set it to 0, which according to the Chaosgroup help page is a special value that removes the restriction on the memory limit and uses all that it needs.
It could be either of these things causing your problem, or something else, but I hope you find a solution to your problem.