I have the same issue. Typically one particular app/channel.
I use the small Express unit, it sits about 4 feet from the wifi AP. I've removed the app and reinstalled, no change, reset the unit, no change.
The app is Fox8 WVUE, which coincidentally works fine on my cell on the same wifi. This is no doubt an issue within the roku.
If there is a limit on apps or memory... it needs to be stated somewhere.