You're welcome.
If it doesn't make any difference when you disable the Avast Shields then it is less likely to be a conflict with Avasts Mail Shield.
Like in the forums.mozillazine.org which mentioned they didn't think a reinstall of thunderbird would make a difference if it is a router issue. The same I guess would be the same for installing a different version of Thunderbird.
I recently had to do a reinstall of thunderbird but that leaves behind your profile settings so when you install it again it pulls in all of those previous profile settings. So you may find no change, even if you completely blitzed it may not resolve it (profile, etc.) you would have to setup all of your accounts again or possible more, loss of email contacts, etc. So there is risk involved there without backing up your data.