You want to keep it at 59.95hz.
The next thing I would check would be your Xorg drivers.
In terminal you can run
sudo nano /etc/default/grub
Tab down to the line that says "GRUB_CMDLINE_LINUX_DEFAULT=" and change it to reflect
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash i915.enable_psr=0"
Then hit ctrl+x to exit, hit the Y key to say yes and hit the enter key to save the current file as is. This is to disable the Panel Self Refresh.
The terminal will revert to normal - run
sudo apt-get install --reinstall xserver-xorg
Then reboot and test.
It's not just Dell that is a real pain about the BIOS updates, sadly...