Dag,
I had the same problem as you a while back and might have even posted on one of your threads. The tank I have that had this problem was the 36 gallon bowfront. Everything I put into it would die within days if not weeks. Mushrooms would wither up and puke guts everywhere. Nothing would live. To make a long story short, I replaced the sand bed, 'removed the skimmer', and let the tank run for about a month to two months this way. After a month I filled a clear sandwich bag from one of my other tanks full of some grape calurpa algae and poked holes in it then sank it in the tank to observe it and see if it would grow. To my surprise it did and I felt more comfortable with sticking corals in there. Now the tank has SPS, Monti caps etc, all thriving. My suggestion to you would be to remove the skimmer for a month, increase the flow, stick some calurpa algae in a sandwich bag and wait to see what happens in about a month. I would say to remove the sandbed but I don't think that sounds like the problem. What was happening with my tank is that the skimmer was just stripping the water column and nothing could live. Hope this helps, good luck.