iphone 12 default mail app intermittently synching with Yahoo account

This started about a week ago. My yahoo mail is not consistently downloading to my iphone 12 Pro (IOS 15.3.1). There are several problems happening randomly: 

--The phone checks for mail and "cannot get mail. The connection to the server failed", OR

--The phone says the mail is downloaded, but it is not visible on the screen, OR

--The inbox goes completely blank even though it says there are messages in there, OR

--Some, but not all, of the messages download, OR

--Sometimes it works correctly.   It's very frustrating.

Fixes I've tried:

--Deleted the yahoo account and re-added.

--rebooted the phone.

--Per yahoo agent instructions, set up a 3rd party password

Is anyone else having this problem? Could the iphone's default mail app be corrupt? I hesitate to delete and re-install it b/c something even crazier might happen. Any advice appreciated. Thanks.