I never had such problems since the very first version of TCO. I used warcraft 1.26 and 1.24e for playing TCO. Note that the campaign was never tested on new warcraft patch later than 1.26. So if you update your warcraft to 1.28 recently, try to roll that back.
Also, I know that it may not be related, but there was a time when I couldn't save my game, because there was not enough disk to save the game. I suggest you to take a look at your disk.