gnus hangs when refreshing Gmail

[David Mann]

I find that when pressing 'g' in the Group buffer, gnus will hang in the
midst of fetching from imap.gmail.com, usually fetching about 90K of
data and then freezing.  I can press C-g and then press 'g' again and it
is able to complete fetching the mail.  After a while (5 mins or
so?) the same phenomenon occurs.

Gnus version is 5.13.  Emacs is compiled from master branch, version is
GNU Emacs 25.1.50.1 (x86_64-apple-darwin15.2.0, NS appkit-1404.34
Version 10.11.2 (Build 15C50)) of 2015-12-20.  Relevant part of my
gnus.el is

(setq gnus-select-method
      '(nnimap "gmail"
               (nnimap-address "imap.gmail.com")
               (nnimap-server-port 993)
               (nnimap-stream ssl)
               (nnir-search-engine imap)))
(add-to-list 'gnus-secondary-select-methods
       '(nnimap "epstudios"
               (nnimap-address "mail.epstudiossoftware.com")
               (nnimap-server-port 993)
               (nnimap-stream ssl)
               (nnir-search-engine imap)))
(add-to-list 'gnus-secondary-select-methods
             '(nntp "news.gmane.org"))

I wasn't successful in googling this issue.  Any ideas as to what is
causing gnus to hang?

Thanks,

David