Disappointed in Thunderbird

Gmail's spam filter is failing me.  Two or three messages per hour are getting through their spam filter, and it's really impacting my ability to use Gmail.

I decided to download the messages to my computer and use Thunderbird's spam filter as a 2nd opinion on whether or not something is spam.

So I download it, set it up, and start training the junk mail filter.

After a long time going through my inbox and marking things as Junk or Not Junk (using the context menu), I noticed that it doesn't matter what I mark it - whenever I mark a message, it becomes Junk.

"Mark as Not Junk" marks messages as Junk!

Great.  I've just trained the spam filter that every legitimate message in my inbox is spam.

Another problem, that's happened to me twice:  I'm going through working on my messages; selecting a message, marking it as junk or not.. then suddenly, all my messages are in the Trash.

I have no idea how they got there.  This happened twice. 

Looks like this release is a dud; what's a good email client (that's not Outlook unfortunately) that has a good spam filter?

Update:  It even gets worse.  The first time it dropped all my mail in the trash, I just moved them back out.  Now, I guess there was gum in the trash bin as the messages are stuck - I can't move them out.