[ubuntu] Ubuntu runs slower

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http://ubuntuforums.org – Hey all, Yup, my ubuntu becomes slow after running it for over an hour. Especially after watching some videos on youtube. The operating system has also freezed once or twice. Had to switch it off via the power button on my laptop. Otherwise, it runs fine. Laptop Config: Dell Vostro 1400, Purchase Date: June,2008, CPU: Intel Core 2 Duo @ 1.8 GHz, GPU: 128 MB NVIDIA GEFORCE 8400M GS, 160 GB HDD SATA @ 5400 RPM (Ubuntu OS is on a 25GB ext4 partition) OS: Ubuntu 12.04 LTS, I have installed proprietary drivers & that didn't help. I use a cooling pad to prevent machine from over-heating. (Hardware)