are you using a dlink router? if so this is no surprise! i personally am using a dlink 514 wireless router (hint: when a store sells a piece of hardware for really cheap... its for a reason!). i had teh same problem ur having now until a few days ago. for months the f*cking thing would work and kill itself every hour or so when downloading large amounts of data, but websrfing fine. had to unplug and replug the router to get thet net back up. it also applies to a few other models because i found the fix info in a forum for a completely different dlink router model.
anywho, enough BS, to fix do the following:
1. write down any settings you've made to your router through the web based config because youre goin to lose them!
2. download the latest firmware for your router, and lots of routers have different "revisions" depending on the manufacturing date, so make sure you get the right one
3. wipe that router clean!!! one thing ive learned is that simply upgrading the firmware does NOTHING to fix a problem, especially in this case. you have to wipe it clean then upgrade. check the manual or support website for your router for info, but for dlink and linksys and im sure most others, you simply unplug the router, hold in the reset button (wherever it is on the box) plug it back in while still holding the reset and keep the reset down for maybe another 10 seconds or so. then go log into your router
4. now that the router is clean, put on new firmware! again, if using dlink, i like to get the english language firmware from the dlink taiwanese site. you can get there from dlink.com and go to the link that directs u 2 different country sites. i like this better because north american (which applies to me, maybe not you, i dont know) versions tend to come out much later and are always buggy! (hey, its like american cars compared to japanese and european!)
5. re-enter settings you saved/wrote down from before all this
6. get your a$$ back on your torrent client and laugh yourself giddy as you download gigs upon gigs of stuff 24/7 without a hitch!
btw, PM me if i wrote this 12 page essay of instructions for no apparent reason because you dont have a dlink router, i completely missunderstood your problem, or you already fixed it and i was WAY off
otherwise enjoy!