As the first step, we recommend you to check the cables you are using in order to connect the Slingbox to your TV source. Most of the times, this issue is related to a bad cable which is not working properly. We also recommend you to provide further information about this issue (cables in use, TV source attached to your Slingbox, etc). This way, other forum users will be able to assist you with accurate information.
Using a s-vhs cable for video connection by chance?
If so; it's either not plugged in all the way on at least one side of the cable has at least one bent pin on one of the sides. Check the cable for 1, 2, 3 or 4 being bent as indicated on the upper left hand side of this diagram on both sides of the cable. Unbed any pin or replace the cable if your unsuccessful in doing so.