This is a known fault with the PRST1. If you read your epub with font set to anything other than 'Original' (e.g. Amasis etc) then italic, bold and bolditalic will be very unpredictable. This is even mentioned in the User Guide.
The current workarounds depend on whether your epubs have DRM or not.
- epub with DRM - no workaround. Read with font set to 'Original' if you want all your bold and italics to display correctly.
- epub without DRM - 2 workarounds, both options require you to edit each epub:
- Open up the epub and add the relevant @font-face statements to its css file. This option can be employed to correctly use any of the T1's extra internal fonts (Amasis, PalatinoNova, Frutiger etc). Read with font set to 'Original'.
You can do this manually or, if you're a Calibre user, you can add the necessary @font-face statements to ExtraCSS and do a conversion to epub. For an example of the @font-face statements used to read in Amasis with all italics/bold intact see this post.
- Fully embed the fonts of your choice inside the epub. Then read the epub with font set to 'Original'. This option takes longer to apply but allows you to use any fonts (TrueType or OpenType) you like.
[Edit:] Seabee beat me to it