Ubuntu 14.04, how to set system priority?

Question

linux

Hello, I installed Ubuntu 14.04 x64 on an SSD, it works fast, there are almost no problems, except for one - when some process starts doing something heavy (for example, a video in chrome, or some kind of php script loads the processor by 50%) - the light starts to glow the load on the hard drive on the PC case (almost without blinking) and the whole system freezes wildly (even the mouse moves jerkily). This continues until the random process finishes its important work, or when it comes out somehow to kill it with your hands. It feels like the system simply does not have a resource priority over the rest of the processes, and this makes it so sad. Some kind of single-tasking. Did someone run into the same troubles? Maybe there are approximate ideas on how to give the system more power over everything else?
PS Processor AMD FX-4300, RAM - 1 bar 4GB, 1600MHz, usually when freezes occur - another ~ 1GB of RAM is free. SSD - Silicon Power Slim S60.
PPS Everything was fine on Windows.

Answer 1

Show

# fdisk -l
# cat /etc/fstab
# free -m

You can monitor I/O like this:
# iotop -aoP

Answer 2

I support the question.
A similar problem happened on different laptops with different Ubuntu OS, c 12-13 and newer.
The question of ssd does not make a difference (I do not have ssd).
According to my observations, wild freezes appear at the moments of using the swap (here I'm surprised, I thought it would be less noticeable with ssd).
The first symptoms - the mouse cursor moves with big jerks and delays.