Repotec broadband router

I recently bought myself a Repotec broadband router (RP-IP515H) and connected it to my @Home cable modem.
When surfing the internet I do not notice any changes in performance, but when downloading performance decreases with about 400%. So instead downloading with 400kB/sec, I’m now downloading with 100kB/sec. The problem is gone when I bypass the router. The number of computers logged on to the router at the same time does not make a difference.
Anyone experienced a similair problem?