I'm sorry to hear that you are now experiencing these "weak or no video signal" problems with your Slingbox 350. There could be a few things that are causing this. The first thing to check would be that your Slingbox is configured to the correct source. You can do this by going into the settings option on Slingplayer, and then select sources. As you are using composite, that is the source that should be selected.
The second thing that might cause this issue could be your resolution. Make sure that your Tivo is on a resolution that is supported for composite. I will place a link below with the supported resolutions, as well as other things that could cause a "weak or no video signal" issue. Hope this helps.
Sling Media Moderation Team