Those levels are very similar to mine. I was having similar problems on R36 until recently (reboots, wireless dropouts) and my SH was replaced. Couple of interesting points from during my own troubleshooting (which was assisted by the excellent support community forum techs):
My SH was showing a lot of T3 and T4 timeouts. Check your counters which are near the top of the same page as signal levels.
Check your SH power supply. Mine was an old one that had a 1A output. I now have a 1.5A output model. Apparently the old ones may be underpowering the SH.
I would highly recommend seeking assistance on the support forums at
http://community.virginmedia.com/