Thanks for the info Dean.
I may try a couple of things, but this computer will be going away at the end of the week, so if it is a local configuration issue, I will know if it happens on the next computer. I was more curious to know if this was a known issue, which it appears to be, but apparently an uncommon one. It doesn't happen on every thread and only occasionally, but I have experienced this for over a year on another IP Board forum that I visit occasionally. I have never seen this issues on other websites or non-IP Board forums. If it happens on a different computer I will troubleshoot farther.
The problem is unpredictable. If I refresh the page the black band disappears. It does seem to happen mostly on picture laden threads.
FWIW, OS is XP w/SP2 and most hotfixes with Symantec AV. We exclude a few non-critical patches that are incompatible with our business applications. OS is dictated by corporate policy. FF 3.6.x is out, but is incompatible with some of my plug-ins, so I have stayed with 3.5.x for now.
I will do another screen capture next time it happens. The band covers the width of the screen and varies in how "thick" it is. I have never seen a black thumbnail before.