What I'm getting from your your post, is that you're having problems with the Slingbox IR.
In this case, and based on the troubleshooting steps that you performed, I would recommend to connect the Slingbox to a different Audio and Video Source as a last troubleshooting step.
If the problem is still the same after that, then you can contact the Sling Media Technical Support to check the options for you to get an IR cable replacement based on your Slingbox warranty status.
To contact the Technical Support Department you can use the following link.
I believe I have the same problem with the IR signal being weak, although I have never needed to use the on screen remote since I purchased the Slingbox 2 years ago, I now have switched cable companys and I now have cable boxes and I can not get the on screen remote to work. When I view the IR signal with a digital camera it is very weak compared to the actual cable box remote.