Slowly connecting to certain servers

[fze]

I'm in passive mode as I'm behind a NAT router.
I have however experienced problems connecting to certain sites, where I'd either getting access after a very long period of time, or not at all.

My status window just shows this and nothing more, until the connection (usually) drops due to timeout:

[L] Connecting to site1 -> DNS=xx.xxxx.com IP=xxxxxx PORT=xxxxx
[L] Connected to site1
[L] Connection failed (Connection closed by client)

I could need some hints and advice on what can be causing this problem.

Thanks