Welcome to the Slingbox answer forum. With the information provided I have a few things I would like to check out. I will post a few articles below.
Another thing that we like to see is the units plugged directly into the wall. You can try removing the serge protection and see if this resolves the issue. If none of these solves the issue you can try to replace the power supply. You can find these at the article listed below.
Hope this helps,
Sling Moderation Team