I have the same problems with mine. I am also running DD-WRT on it. From all the reports I've read, this wireless problem seems to be the norm with this router and firmware. I usually reboot the router to get the normal performance back.
rov3rt, are you using wireless-n, g, or mixed? I had heard speculation that it was related to n speeds, although I don't want to change mine to g, since that lowers my internet connection speeds by 30%.
-- A question for anyone from Buffalo... Is this problem being worked on for the next release of DD-WRT.
I'm not sure how much longer I can put up with this before I need to find another solution.