Ubuntu 14.04.4 LTS on Pentium4 with 3G RAMB

Hi,

Needles to say I'm new to Ubuntu and have a question.
I've installed recently Ubuntu 14.04 LTS and I'm quite ok with how it works except for one thing.

Every time I'm starting a Mozilla session or start to stream a movie (not HD) from the plex media server (plex.tv) installed the CPU is 100% directly.
The PC is still working but I would not except a single task to drive my CPU to 100%!!

The PC HW I'm using is a bit old but well above the reqs specified to run Ubuntu.
Is there anything I should do to improve this?

Below is some more detils on my system
GNOME: 3.8.4
Keel: 3.19.0-56-generic
CPU: Intel(R) Pentium(R) 4 CPU 3.40GHz
RAM total: 3009 MiB
Motherboard: GA-8I915P from Gigabyte (a bit old to be honest)
Graphic card: NVIDIA GEForce 210, manufacturer is Asus

BR Adrian