thanx for the reply technut!

i download mp3 (ftp running with xampp on home pc) over wifi at external slot (kingston 8GB sdhc cl.6). with maemo-ftp, opera and microb, all with the same result. internal slot is original nokia 128mb card, it is now all used for swap. after downloading a short time (about 30 seconds) the speed decreases down do 140kB/s. after a reboot i have the full speed (~600kB/s) back for a short time. the same behavour i have @ internet, so it isn`t just a problem with the ftp server in home network. mem and cpu are not working on full capacity. router is asus wl500gp with openwrt on it.
should i reflash the n800? i spent a lot of time for installing all apps so i`m not very happy about this way, but if i find no other solution...