Since your Slingbox Classic seems to affect your TV set, it means that you need to check all the network connections. There should be no interference in order to affect the TV signal.
If you are using a cable box, you must connect the coaxial cable that comes from the wall into your cable box. Then, from the cable box outputs, send the connection to your Slingbox inputs. Finally, from the Slingbox outputs, send the connection to the TV set inputs.
This way, your Slingbox Classic will not affect the TV set at all.