Excel sheet not scrolling

Hi Everyone. I can usually find the solution to anything but this one has me stumped...

I am experiencing a very specific new issue with scrolling in Excel:

• I can't scroll "multiple lines" with the mouse wheel in Excel
• I can scroll "one screen at a time"
• It only affects Excel
• It only affects previously created sheets - a newly created sheet was not affected
• Scroll lock, freeze panes and other known issues are NOT at fault
• I can scroll horizontally using shift control
• It's not the mouse or drivers - I re-installed both - and used a different mouse.
• I rebooted etc. - nothing.
• I have searched extensively for an answer to no avail - it has to be a previously undocumented problem
• Looking forward to an answer!!!