I can confirm the issue you are reporting - WinXP Pro, SP3; FF3; SW 188.8.131.526
This latest version of SW (596) was released with a fix for a scroll bug in #text members, but yours seems to be a new/different issue. Are you reporting that this didn't occur with build 595 (the initial SW release co-inciding with the D11.5 release)? What about authoring - do you see it there?
Can you create a smaller SW file with only this scrolling text member in it, and place it and the unprotected Director file on the web somewhere?
The issue has started with Shockwave plugin updated to Director 11.5
compatibility. I think I can be sure there was no problem of this sort
with Shockwave plugins compatible with Director 11 only.
I will prepare small movie you ask me for and put it on my server in an
open format but I need a day or two for this.
I just tested with build 184.108.40.2065 and it occurs there too.
You need to submit a sample to Adobe and kick up enough of a stink that they take it seriously. Contact Allen directly - email@example.com - but if you can provide an online sample I will post a link to Direct-L where other users can also clamour for a fix (more noise and visibility might provide greater traction).
Thank you, Sean - I will follow that tomorrow.