Under employment of the Net graph I have determined that the DL speed slows down every ~28 seconds -> moves back on normal rise after a few seconds. This problem seems to appear only, when DL-Speedlimit is used ("Obey download speed"...).
http://www.0xFF.de/forumdata/fxp_dlspeed.jpg
* FlashFXP v[3].[0], build [1010], [X]registered, [ ]unregistered, [ ]pirated
* OS [X] WinXP, [ ] Win2K, [ ] Win98, [ ] WinME, [ ] Other
* Running behind NAT/router [X] Yes & Model [Netgear WGT 624], [ ] No, [ ] Not sure
* Running firewall [ ] Yes, Name [ ], Ver. [ ], or [X] No
* Running Antivirus [X] Yes, Name [F-Secure AV 2004] or [ ] No
* Network [X] xDSL, [ ] CABLE, [ ] Dail-Up, [ ] Other