I think it is time you made a concerted effort to get some assistance from Dell.
Tell them you have sought advice on various forums, but the problem is unresolvable.
It might take time to finally get through to someone who can help, but as Klown found out, it was worth it. He is about to receive a replacement.
And it doesn't matter about whether the device is in warranty or not, in my opinion.
If you can't get through to someone, carefully pack the device and send it back to Dell by registered post (or deliver it), with a letter explaining the problem. Make sure they have to sign for it.
Whilst I have imported Cowon and Archos devices from the US (therefore without warranty), I don't think I would risk on a Streak - unless the price was exceptionally cheap, like $99 unlocked. Because if it went feral, Dell in Australia would not be obliged to assist.
I doubt removing apps. will help, the problem may be a file you downloaded, though as you say you have checked with AVG.
But even a thumbnail photo in backup, with the original deleted, seems to cause Tubemate to consistently pick up the thumbnail in backup, instead of from the video it downloaded. Also very frustrating, and difficult to fix. And an antivirus will not pick such issues, which are not malware related.
It is important to write down the date when things started to go wrong, and then use a File Manager to remove files with that day's date. And you need to search through every folder looking for them.
However, you device may simply be faulty. So contact Dell and try and get a replacement.