I got curious problem on my c1802.
I updated IOS from 12.4.15t6 to 15.0.1M and then back to 12.4.24.t2 and meny times I change ADSL firmware too.
Noticed that my original ADSL FW 3.0.14 was in new IOS 15 builds useless because established connection was only 7Mb instead of 15Mb in down so I try newer firmware 4.0.3, 13, 15 and still same problem, then I downgrade IOS to 12.4.24T2 and still same, I find out that I must use 3.0.10 firmware and got full line speed 15Mb/766kb all newer had hald DL speeds. Now I running on 12.4.24T2 and 3.0.10 FW and have problem with downloading files using FTP. It starts about full speed 1.0-1.4MB/s and skips to 300KB and up down and finaly it after 5 sec leave about 600KB/s, measured from my ISP FTP server on Filezilla client. The downloading using HTTP not have so big fluctuation and run on 1MB - 1,1MB/s
ATM MTU was 1452 and BVI was 1412 I changed to 1492 / 1452
Looks like some buffer overflow.
Thanks for any help.