My iPhone deletes my yahoo email on its own! (Disappearing emails)

I use push email with 3 different yahoo accounts. Since buying the phone, it will occasionally just delete email off the yahoo server without any input from me. The emails are just gone - not in the deleted emails folder, trash or anywhere. I replaced the phone, and the problem continues with 2 separate yahoo mail accounts. Not all emails are deleted, just random ones, such that over the course of a week I've probably lost 5-10 emails. All my settings are set to never delete email.

I have a few examples of deleted emails, as I also occasionally download emails into Mail on my PowerBook (set to leave email on the server, which works normally). I've been doing this for years with a Blackberry and have never lost an email.

Apple says it's yahoo's fault, and I haven't yet heard back from yahoo.

Help!