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I thought I would share my experiences and eventual solution by a process of elimination.
A bit of background in that the slingbox at my parents house and I live in the US, so there its not always easy to check things at both ends.
Working with a friend we checked to make sure the unit was getting enough power with a voltage meter and it was.
I set up a manual direct connection as described in the link above, but while a good idea it did not solve my connectivity problem.
Again and again the connection would drop. We tried leaving a computer running 24/7 to ensure the connection was not broken. Still did not work.
Finally we worked out that the modem was faulty and the connection dropped. We replaced the modem and that seemed to work in the main but still occassionally it dropped or the set top box froze and it needed a reboot.
Now if there is a problem I can check things to see what is wrong.
1st I now know my routers remote access ip address and so I check if I can access it. If I can then I know the internet connected properly.
2nd check the virgin media site to see if there are any outages in the area https://my.virginmedia.com/faults/service-status
3rd log onto slingmedia support. There it might tell you the fault number as they ping the slingbox regularly.
4th Now as you know the IT solution for everything is to reboot (or swith off the modem and router, wait a minute then switch back on). Now that ain't easy if the parents are away or its in the middle of the night given the time difference in my case. So I dug around and found this solution. I bought a phone switch http://www.technotrend.co.uk/pages/ss_products/phoneswitch.htm I have my set top box, dvr, slingbox, router and modem all connected to the phone switch. Now if all else fails I can dial up the switch and remotely switch everything off, wait a minute and call back to switch it on again. So far I have no further problems with accessing and streaming from the slingbox.
Hope this helps.