Thanks for the suggestions. I'm working on updating video and sound cards right now. But it is strange that I haven't had this problem until now, when I'm so close to the end.
I do have the Loch Ness version of the game. I'm running it on a Compaq Presario 7470, Windows 98, Trident Blade 3D/MVP4 (63) video card, ESS Allegro-1 sound card. There is nothing running in the background. I haven't tried to start a new game yet but I will try that after I have done some updating.
Whose cruel idea was it to put an "s" in the word "lisp"?