Hi JimC,
Yes, the gamesave mechanism in SH3 can be a PITA, it can be temperamental. However there are ways to minimise the effect it can have. One thing I do, is to make two saves per game, under different names, I usually find that if one fails to load, then the other save normally works for some reason. I make these different saves a minute or two apart. Just my little workaround that seems to work for me.
Although there could be a number of reasons why the gamesave is victimising you.
If you would post your machine specs, and what supermod and/or other mods you've enabled or disabled, plus if this is a recent development or has been ongoing for some time, this would be a great help to the computer savvy among us.
Hope you find a solution soon.