Hi USP45,
Could this issue always be reproduced on these pages (containg InfoPath form) for these particualr users?
Please try the following see if it helps:
From IE9 side, try to clear IE cache, use compatibility mode, test on another machine, press F12 to check if there is any error like javascript.
From SharePoint side, check-out/check-in and publish the problem pages, recycle SharePoint site if possible, also may check ULS log when issue occurs.
Thanks
Daniel Yang
TechNet Community Support
