My iMac drops the internet connection but mantains the wi-fi network.

Question

I have a late 2011, 2.7GHz Intel Core i5, 27" iMac with 4GB Ram. When I power up the computer everything goes great, but after 3 or 4 hours (top) the internet connection stops working; and by this I DO NOT MEAN that the network utility turns off, or that it doesn't detect any netwoks. On the contrary,  the computer stays connected to the wi-fi network and detects everything, but it simply has no internet connection; it even stops communicating with my iphone through itunes (with the wi-fi sync option), and crashes LAN games when i try to play at home. When this occured for the first times I just turned off and back on the network utility, but the problem continued and I had to keep turning off and on the network (it wouldn't stay connected for more than 5 minutes). I  checked with my laptop (the last alluminum macbook ever available, as well as with my mom's black macbook) when the internet stopped working in the iMac (just to check that there were no issues regarding my ISP) and the internet connection just worked great in it, and it didn't drop at all! It also worked perfectly with my dad's windows XP dinosaur laptop.
At the beggining I thought my wi-fi router had gone bad, so I changed it for a brand new wireless-n ready router (it turned out that i wasted 180 dollars, because this wasn't the issue, and I had already trashed my old router). At first, I used the wireless-n feature, but other gadgets stopped working with this type (a play station 3 and a nintendo wii). I then configured it with a mixed mode, and everything has worked perfectly (including both macbooks, an ipad 2, an iphone 4s, 4 blackberries, a play station 3, a nintendo wii, 2 network printers, 1 network scanner, a 3TB network hard disk and 3 led tvs that come network ready, along with their respectives network ready surround soundbars).... EXCEPT FOR MY IMAC! So DEFINETELY MY ROUTER IS NOT THE PROBLEM.
I've been an avid apple user for almost 10 years now, and this is the first time I encounter this type of problem (it's my first iMac), so I'm pretty much out of logical options. I've already moved my router closer to the computer, I moved my wi-fi connection to the top of the list in the network utility.
I even had to re-write this whole message again because i clicked on "Post message" and it had no internet connection, so it displayed a no-internet alert in my browser.............
PLEASE! HELP!!!!
PS: all my apple gadgets are always up to date.

Answer 1

I FINALLY SOLVED THIS ISSUE!!!!
New routers have "mixed" modes for your network; this means that it will automatically choose the best option between wireless-B, wireless-G and wireless-N network modes. I used to experience this issue using the 2.4 GHz. mixed mode wireless network.
Thankfully, my router can create 2 networks: a 2.4 GHz. network and a 5 GHz. network.
When I randomly changed this value to "use only wireless-B and wireless-G" trying to fix an issue with a wi-fi expander, the iMac wi-fi  issue dissapeared! I tried it with wireless-N only, but it didn't work.
So, finally, I have a 2.4 GHz network with only wireless-B and wireless-G modes which works PERFECT with the iMac, and a 5 GHz. network with a mixed* wireless mode for everything else mode.
*PD: by "mixed" mode it means that the router will automatically choose between B, G and N network modes as it fits better.