It could be nothing to do with the router, but the power supply.
See if you can borrow a simlar PSU (Netgear all seem to use a similar PSU) and try that.
It may also be a heat issue since you are experiencing problems when dowloading at full pelt. Ensure all vents are clear by using a vacuum hose over them and making sure they are not blocked by something close. 
It appears to be a common problem although I have one that gets hammered and is fine.
http://www.instructables.com/id/Netg...-DG834G-and-s/
I agree with Kymmy. Soldering/resoldering/replacing surface mount components could lead to worse problems if you are not upto it. I am also trained in doing it and it's alot different to changing leaded components.
If you think you are upto it then you need some of this.
http://uk.rs-online.com/web/p/solder...ables/3143469/
and a good soldering iron.