Major problem with fonts and pdfs

I'm experiencing a strange problem. When I view pdfs using Preview, many of
the characters of the fonts are either missing or (more commonly) replaced
by black boxes. This happens with pdfs I've created on this machine and which
were working perfectly (the pdfs were created using TeXshop). It also happens
with downloaded pdfs. What is also strange is that if I quit Preview and re-open
the same pdf file, I get different patterns of missing/blacked out characters.

If I try printing the file to a printer (HP LasterJet) the character mess remains.

If I try opening the file in Adobe Acrobat, then the file is perfectly fine.

I've tried deleting my plist files, the problem remains.

I've re-installed Preview from the OS discs using Pacifict - the problem remains.

If this were a problem with one or two pdfs, I would put it down to badly constructed pdf files. But this now occurring with about 60-70% of all
the pdfs on my machine!! And as I mentioned above, even ones I created and
that had been working perfectly a week or so ago.

If anyone has any thoughts on why this is happening and how to fix it, I would be most grateful. I'm a researcher and I have a large number of pdfs
for my work that I can no longer read or print!

Many thanks,

Adrian