Can anyone suggest a fix for this problem?

I am running FSX on a Windows 7 pc. Everything was fine, but now the "View Zoom In" ( = ) control function has stopped working; doesn't do anything. "View Zoom Out" ( - ) continues to work. Similarly, the "Backspace" function is working fine, returning the view to the base position.

I have searched various strings and tried fixes; Backspace key (to "reset"); "Ctrl and =" keys; "spacebar" (again to reset), but no change. Finally, I have looked in the fsx.cfg file but can't see anything obvious that might be causing the problem -- but I don't really know what to look for.

Any advice?

Thank you,