wifi problem on imac

Hi everyone, I have an imac running on OS X 10.7.5 i bought it years ago. My problem is that when i connect my imac to my wifi network my imac works perfectly but after 3mn of surfing the browser tells me that i have no connection but it is connected to the wifi network after that i disable the wifi and activate it again (on imac) and the same problem happens to me .. this really bothers me .I really need your help guys

PS. my phone and my laptop work completely fine on the same wifi network . Im sure that isnt on my router.