Illegible bitmap fonts

Yesterday some of my bitmap fonts started to look very unclear when I was making icons in Photoshop 7, and when I say unclear, I mean illegible. The 'W' is showing as a solid square, and the 'O' is a solid round for example. The fonts affected are 04b_24, 04b_19 and Ernest. Oddly, they actually look clearer with anti-alias turned on, though still not as distinct as they should be. I've double checked all my settings, tried deleting and reinstalling fonts, uninstalling Photoshop and reinstalling it, and system restore. I'm now out of ideas, and feeling stuck without some of my favourite icon fonts. Has anyone any suggestions for a fix, or what the problem might be?

Cross-posted to fontaddicts.

ETA: Solution provided (though I still don't know why it happened). In case anyone else gets the same problem, switch the font size from an even number to an odd number, or vice versa. Thanks to engel_x for that fix. Inbetween font sizes also work, such as 7.4 for a font that you'd normally use on font size 8.