Same issue here with my SLINGBOX PRO, started right after the new year.  I have a lot of experience trouble-shooting so I have tried everything.  Past issue I had with this box was the speed drop issue which I fixed with a firmware rollback.  Tried that again here and did not help.  Tried a different router, no help.  Cannot get video through S-video, composite or coax to work.  Sometimes I get sound and a choppy / distorted barely visible image, other times it times out and cannot connect.  The player says "No Video".  I have tried 3 different players on Mac, Windows, and Iphone.  None work. 

Another thing I tried was to make sure my Fios box was outputting the image with HDMI cable hooked up by taking the S-video, composite and coax cables to a TV, all three work.  One thing I did notice was that when I ran the composite and s-video first to the slingbox, then out to the TV, I did not get an image on the TV.  I did using the Coax but still, none will show a image on the Slingbox.

I am at the point where I am going to try replacing some of the capacitors like the other post suggested for the SB Solo but I am not very confident.  None of my caps look blown, I did some testing of them with a multimeter and a few gave me some funny output when they were still connected to the circuit board, but behave fine once I removed them.  Who knows. 

Very frustrated at this point and really hating Slingbox and its lack of help on all of their issues.  They just constantly deny things are their fault.  The speed drop issue was a total nightmare as well. 

Ill report back when I am done fooling with the new capacitors.