WR841N stopped working properly

Hi. I've had a TP-Link WR841N 300mbps router for about a year, with my 10mb internet connection. My desktop (by cable) received the full 10mb, and other computer connected via wireless also did.
I upgraded the connection to 30mb on September 1st. My desktop (connected through cable) receives the 30mb, but for some reason the other computer via wireless only receives 1-7mb, sometimes even less (pretty unstable speeds)!

It was all working properly before the 30mb upgrade... I don't think that's a reason to make the router stop working.

I've been trying to fix it for over 7 hours, and no success. Firmware upgrades, custom firmwares like dd-wrt (which messed up my router, had to revert to the stock firmware), changing values and options, nothing works.

What can I do (besides buying another router of course)?