The only thing I have found is that there appears to be an issue with the NVIDIA driver, linux emulation, and sound on FreeBSD. I have had freezes playing flash videos(less rarely) and with skype (very rarely). Using GNASH solves the crashes in flash but comes with its own issues.
Sorry, but as far as KDE, I am just not aware of anything in there that would cause your system to crash and reboot. Suggestions would be to try a different nvidia card if you have one or try the NV driver (which will most likely not crash at all but kill your CPU cycles).
The only things I can think of would be how non-standard you are. What is in your /etc/make.conf? Which compiler are you using - CLANG or GCC ? What ports do you have installed? Which non-standard options did you use in your ports?