Wifi power saving can cause the N900 to become very unresponsive to incoming traffic such as SSH and SCP with some types of routers.

You can keep the Wifi active while you are using SSH by opening an xterm on the N900 and leaving a ping command running that is pinging your router. You need to be root to run ping.

You can also go into the Wifi settings for that connection and disable power saving, but that will cause the battery to run down quickly when you are connected to a Wifi connection with power saving disabled.

The power saving option is fairly hard to find. You need to press "Edit" in that connection's settings page and press "Next" three times to get to the last page then press "Advanced", select the "Other" tab and set "Power saving" to "Off".

I hope this helps.