Old forum, I know, but maybe someone encountering the same problem these days, too.
I have a DIR652, but the same problem (luckily, I could improve the situation).
I'm behind double NAT (ISP is doing NAT, with rate and connection limit) and my home router is doing NAT, of course. So connection quality was really bad in some cases, with dropped ACK and random packet drops in TCP connections. Typically, applications which are opening many TCP connections to download did suffer (not really talking about torrent, but things like youtube, gmail which are accessing content delivery networks, e.g. akamai).
So, how I could improve my situation and keep SPI and other firewall functions ON: I enabled WAN traffic shaping and Automatic upload speed detection. Bad luck, if this is not available in your models...
Hope this helps some. Good luck'