...we tried everything and nothing worked.
Hehe... I love this answer. If you tried
everything and
nothing worked, then what else is there to do? There's nothing else to try, and nothing can fix it. You'll have to live with it, apparently.
...or, we can be less dramatic and really start drilling down and find out what the problem is. Let's try some of the following things:
1) Boot from an alternate source -- a copy of Mac OS X installed on an external drive, etc. Does it do it then?
2) Create a new user account and log in under that new user account (as mentioned previously in this thread). Does it do it then?
3) Reset your PRAM. Instructions for doing this are easily found with a Google search (or a search of this site). Does it do it then?
4) Connect your computer to a different monitor. Does it do it then?
I think this is inherrent in this 10.5.7 OS.
While it may be 10.5.7 that brought about this problem (or something that happened during the update process), this problem is most definitely not inherent to 10.5.7 -- I have a 2009 Mac mini, same as you, and am running 10.5.7 with three user accounts. None of them exhibit this problem. While that doesn't help you much, I'm just saying that not everyone is experiencing this problem, and, therefore, it is
not inherent to 10.5.7.