Mail and keychain problems when upgrading to SL

I have tried twice to upgrade to SL from 10.4.11.
Initially everything is smooth as glass, fast, no issues at all.
After I open mail everything works fine. After closing mail; the next time I open it and try to get mail the little boxes asking for my password shows up on 3 of the 5 accounts.
I add the password and I can get mail.
However if I check the box to save the PW to the keychain, mail stops working. In the worst case I loose all my mail in the account where I asked for the PW to be saved.
Needless to say I only did that once.
I do NOT get the use mail 1st time message until I loose my mail. Then everything is gone
I have tried to remove and re-add the offending accounts both in mail set-up and in thew keychain.
" " " " " the mail.plist.
" " " " " " " " to the desktop and re-placing it.
I have talked to apple support and nobody has a clue. 1st time was Aug, '10, then again this weekend.
Still the same problem.
On the positive side Safari was working again, and Aperture "flew"
So I really want to upgrade.
The people I have talked to who have upgraded from 10.4.11 to SL do not seem to have had a problem.
Everything works fine with Tiger, except Safari and Aperture is slow. Firefox is also slow, but Safari doesn't work anymore.
Being a former techie (retired now) I have checked, DBL checked and tripple checked. Permissions are fine, althoug after the upgrade there were a couple that I fixed.
I am back on Tiger, Good thing I backup frequently.

Please can anyone assist?????
I am going to try it again, but not until I get some productive feedback

Thanks
Hanne333