ITunes crashing/10.6.8 Combo image not recognized

Question

I have been having the widely reported iTunes store crashes, as well as persistent permissions errors, slow performance, and difficulty buring DVDs from the Finder. Files can't be found, etc. although they are there and although I'd repaired permissions before trying to burn.
I hoped that using the 10.6.8 Combo updater would repair these problems.
It is a two-month old MacBook Pro running 10.6.8 (updated via Software Update).
I have downloaded the Update Combo from the Apple site (http://support.apple.com/kb/DL1399) three different times, but each time I prepare to run the update, when I click on the dmg file, I get this error:
I have downloaded the update at quiet times (including at night) and have no reason to believe there have been interruptions that might have corrupted the file. I am on a T1 connection.
Is there a different source for the update image? Is there something else I can do? I have repaired the disk from Disk Utility on the install disk, have repaired permissions also from the install disk and from the mounted volume (including in Safe Boot). Each time there are many, many errors that are repaired.
--Sarah

Answer 1

Replying again. Many other crashes, in Safari, Skype, iTunes (store), iWeb, Dictionary. Frustrated, I went to the Apple Store today and had a session with a Genius. He could not install the 10.6.8 Combo Update on my machine from an external drive. Then he checked software updates and I saw a THunderbolt update I hadn't seen before. Downloading that allowed me to download the 10.6.8 Combo Update, and installing that update has apparently solved the problems.
So the key is to install the updates as they come out. The Thunderbolt update was billed as being about "stability" and it seems to have resolved something in the 10.6.8 update that I'd done via Software Update.
Resolved.
Message was edited by: Sarah Pradt--added Safari as a program that had been crashing