randomly freezes. system cannot power of.

when i surfing on the web (with chrome) or reading pdf or watching videos on youtube, system freezes randomly. also when i shut down or restart the computer system can't power off.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: linux-image-4.2.0-34-generic 4.2.0-34.39~14.04.1
ProcVersionSignature: Ubuntu 4.2.0-34.39~14.04.1-generic 4.2.8-ckt4
Uname: Linux 4.2.0-34-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
CurrentDesktop: Unity
Date: Sun Mar 20 07:42:56 2016
InstallationDate: Installed on 2016-03-19 (0 days ago)
InstallationMedia: Ubuntu 14.04.4 LTS "Trusty Tahr" - Release amd64 (20160217.1)
SourcePackage: linux-lts-wily
UpgradeStatus: No upgrade log present (probably fresh install)