My Browser is inconsistent

I use the following browsers: firefox IE and safari(pc edition). though every checks done proves that my internet is active, i find it difficult to browse at times (even while my messengers are running, and downloads in progress) the site barely loads and if it does it takes a longer time, its not the same all time. on certain situations it loads so normal. Now i’m about to share the internet with 6 other computers at my workplace. what should i do to prevent my browsers from breaking. please help..