I'm on level 22 and I can't save manually any more - my save just goes over the last one. What do I do so I can save again? Do I have to delete saves? If so, how do I do it?
You should check your hard drive space, which you can do in the system settings on the XMB. Then you should try to free up some space. This you can do by deleting downloaded content, and installed game data.
In any case, you can also try deleting save games. Go to the games menu, go up, find your saves, find the fallout folder, see the saves. Press triangle, do to delete. Do this as often as you like.
Well, you should always check your hard-drive space, but I doubt that is the problem simply because you would have experienced similar affects in other games.
You can only have a limited number of save files I believe (I think it's 1000, so I somehow doubt that's the problem either). It could possibly be the ps3, have you reset it recently? But to delete save files, go to save files at the top of game scroll, open it and press triangle, and a right sidebar pops up with a delete option. Please be careful if you decide to do that, I accidentally deleted one of my brother's saves once trying to remove a file to a game I no longer had XD.
Fallout is also just a really glitchy game (no offense to creators, I love the game, it just has some problems), so I suspect it's a glitch. I have gone through the game 3 times, and 2 of those times there was a serious glitch, such as once it took all my items and sent me back to the beginning, with everyone in that town dead (which I took their side against the powder gangers, so I don't know why). Sorry, I wish I could help more :/