I find that mine will want to turn off the transmitter as quick as possible with power saving mode. If I'm to settle down for a longer session on WiFi, I'll plug in and let it run in normal mode, it's much more responsive and doesn't drop off.
You can just turn off power saving mode too, if you're not worried about conserving battery life.

Sometimes there could be an IP conflict for the router too, if there's too many devices on and off the WiFi. We sometimes experience issues when our son has been on his XBox.
I use WiFi Manager to administer my wireless on the phone, it's a good app.

Also, make sure you are running on Gingerbread 2.3.6 (latest update) to the system software.