Do you by chance have a DAW? You have no signature details so no idea what else you have to work with. Was wondering if you had a DAW you could import a midi file into? If so, you could export your Sibelius project to a single midi file, then import it to a DAW and then see if that DAW responds the same.
It still sounds to me like an audio driver issue. A hard drive not able to keep up would cause this too but if it worked in XP64 with the same setup then you can pretty much throw that out the window.
Try turning off any options in Sibelius such as "reverb" etc to increase performance until you get this figured out. I stopped using Sibelius a while ago as my DAW. Now I only use it for scores because it was to much of a CPU hog. That and it was the worst product I ever used to record in live parts. It would always miss notes, screw up passages etc.
Maestro2be