No, I think it's a UBB problem.
You see thread list A,B,C,D,E,F
You Click B, read it, hit back
And see thread list C,D,E,F (A, B is missing)
refresh and it's back.
The browser never saw or cached this C,D,E,F
I see it myself and I use many boards and only on my UBB site I see it.
Shutting off the caches in UBB definitely helps but does not get rid of it completely.
