Brothers newly built computer runs Saints Row IV on super speed and stutters on Youtube videos

[Jauho]

As the title says, my brother got a newly built computer last weekend and he's been having a lot of fun with it. But whenever he play Saints Row IV, it seems that the game runs really too fast (i dont know how to explain it, like Saints Row 2 did a while back on modern computers). Also his youtube videos stutter like crazy, what happens mostly is that the audio keeps going but the picture gets stuck.

What could cause those problems and how to fix them?

Here are his specs btw
AMD FX-4300
AMD Radeon R9 280
8GB RAM
1tb harddrive
amd 970 am3+

Thanks in advance!

[editline]27th October 2014[/editline]

oh and he uses some Samsung 42" HDTV as a monitor, connected through HDMI

[Jauho]

while gaming the temps of his cpu and gpu are 30c and 35c respectively

[editline]27th October 2014[/editline]

Oh and TF2 seems really jittery for him too. CS:GO is the only game that runs fine