Stalled message move in MacOS10.13

I have an iMac about 4 years old running macOS10.13.3 (High Sierra) and Mail11.2. Since (and only since) I installed High Sierra, moving messages from the InBox to folders I have set up stops working some time (variable) after opening Mail. The only solution I have so far is to close Mail and then reopen it. This may (or not) be associated with an intermittent downloading of 1000's of mail messages of some undefined provenance, which may be slowing or stopping the move message operation. Finally, Mail makes many (sometimes dozens) of copies of draft messages and keeps them after the messages are sent, but this behavior started a few OS versions ago and is not specific to High Sierra. All my mail comes to Mail via Gmail but I have my own domain name.