How to fix GMail time stamps in Outlook?
Solution 1:
A Google forum post directs to Gmail's help that has this behaviour documented. ("This is a known issue, and we appreciate your patience as we work to resolve it. updated 9/19/2008")
According to another Gmail support page, Outlook Express shows by design the time you received the messages locally, not when the server got them when using POP but this shouldn't be the issue as you have Outlook and used IMAP.
There is also an MS Help & Support article about the 'incorrect receive time' issue. It might not help in your specific situation but it doesn't hurt to see it through.
And when talking about trial-and-error, does it make any difference if you import the messages to Gmail via IMAP?
Solution 2:
I think this is because they are two different time fields. Outlook displays one and Thunderbird the other. Can you paste the headers of an email downloaded using the original Imap and one after using pop3?
If you can still use imap (albeit slowly) then you could connect to both servers using imap and copy or move the emails across thus preserving the timestamps. I appreciate from your question that this might not be possible but I thought it was worth mentioning