It will be easier to help you if we knew more about your hardware. iMacs have been around since 1998 in four distinct hardware versions, each with its own characteristics and issues when troubleshooting. At minimum we need the processor type to identify your model and your version of the Mac OS. Both are available by doing "About this Mac" from your Apple menu, to get a screen like this:
If the processor info includes the word "Intel" we need to get you moved to another forum. This forum is for older iMacs built before 2006. From your symptoms, it sounds like you have an Intel 27-inch iMac. Lots of reports in the other forum about that version dropping wifi signals.
Currently Being ModeratedJul 14, 2012 11:54 AM (in response to Jamo The ipod)
OP has a known hardware problem, posted in the wrong forum, and has been successfully refered tthe proper forum setion. Resets/restarts won't fix this kwon issue.
From other posts, I get the impression that you are not reading the entire thread before posting. These forums are more enjoyable and informative if you slow down and smell the roses.