Is the process to make them freeze repeatable?

No:
try using strace's -p option. If you have an occasional frozen process, it's probably not a serious problem. Try to come up with something that repeats the problem.

Yes:
Try running strace on one of the processes, and watch it from the beginning and see if it is waiting on the kernel. If it is, use the 't' sysrq function, find your frozen process in the output, and post it. Always use CONFIG_KALLSYMS