After playing fifteen minutes or so, when i try to save i get a message "error saving game: unable to save game"
It's gotten to the point where i save every five minutes and occasionally have to restart just to redo the last few minutes of gameplay. for instance, i just killed the deathclaw nest in the boneyard, only i didn't!
oh, and since it deletes the save when you overwrite, i have to leapfrog between two saves. that's not really relevant, i'd just like to make it clear this is so regular i've developed a system.
i really hope there's a fix for this...

sounds like some files are corrupted. copy your save files to another directory and try reinstalling fallout. put your save files in the new installation and load them up. if that doesn't work, then it could be the save files that are corrupted, which would mean you'd have to start a new game.