I don't know about bug-fixes but for me 4.5.3 (probably also 4.4 but I can't say for sure) had some regressions on athlon-xp. On a machine that passes memtest over night and almost never gives me any problems with code compiled with 3.6 and 4.1 - 4.5.3 segfaults randomly when building itself on the second stage (after bootstraping).

4.3 seems to generate stable code and I'll stick to it for a while.

I started noticing problems when recent 32bit images from ubuntu and gentoo freezed the PC.