Constant IMAP timeouts/dropouts

[H.E.]

Hello,

   I've searched and searched for a solution to this problem but 
continue to come up empty.

   I have a very generic setup.  CentOS Linux 7.3.1611 running on AWS.  
   I installed the QMail toaster from here: http://www.qmailtoaster.com/
  ( I had been using Bill Shupp's Qmail Toaster for years and years, but 
he stopped supporting it.)

   As such, I did have to convert my Courier mailboxes to Dovecot (no 
real problem there).

   I'm using Thunderbird client on Windows and Mac.

   I have one user (me) using two vpopmail domains with a total of three 
mailboxes.    Each mailbox has about 20 folders which I tag in 
Thunderbird as "When getting new messages for this account, always check 
this folder".  I might have two or three client machines (at different 
locations) running at once (desktop and two notebooks) all trying to get 
mail.

   The problem is I can't go 5 minutes without Dovecot timing out on me, 
returning an error in Thunderbird: "Login to server failed" and asks to 
"enter new password, cancel, or retry" options, or just hanging.

The only way to get Dovecot back is to bounce both dovecot and the 
network via:

/bin/systemctl restart  dovecot.service ; /bin/systemctl restart network

Then I can resume normal activities for a while.  It got so bad that I 
put the above restarts into a cron job running every 5 minutes. But even 
then the problem persists after a couple of minutes. There's nothing 
else running on this box.

I've already increased both
mail_max_userip_connections = 8000
and:
protocol imap {
   mail_max_userip_connections = 8000
   mail_plugins = " quota imap_quota"
}

By the way, the the identical mailbox and client setup, I never had this 
problem with Courier IMAP.

My dovecot -n is pasted below.

So I'm at a loss on how to fix this.  I have tried adjusting the number 
of cached Thunderbird connections, and that doesn't change anything.

Any help or pointers in the right direction to keep Dovecot connected 
would be most helpful.

Thanks,
-hank

Here is my dovecot -n

# 2.2.24 (a82c823): /etc/dovecot/dovecot.conf
# OS: Linux 3.10.0-514.26.2.el7.x86_64 x86_64 CentOS Linux release 
7.3.1611 (Core)
auth_cache_size = 64 M
auth_debug_passwords = yes
auth_mechanisms = plain login digest-md5 cram-md5
first_valid_gid = 89
first_valid_uid = 89
log_path = /var/log/dovecot.log
login_greeting = Dovecot toaster ready.
mail_max_userip_connections = 8000
mail_plugins = " quota"
namespace {
   inbox = yes
   location =
   prefix =
   separator = .
   type = private
}
passdb {
   args = cache_key=%u webmail=127.0.0.1
   driver = vpopmail
}
plugin {
   quota = maildir:ignore=Trash
   quota_rule = ?:storage=0
}
protocols = imap
ssl_cert = </var/qmail/control/servercert.pem
ssl_dh_parameters_length = 2048
ssl_key = </var/qmail/control/servercert.pem
userdb {
   args = cache_key=%u quota_template=quota_rule=*:backend=%q
   driver = vpopmail
}
protocol imap {
   mail_max_userip_connections = 8000
   mail_plugins = " quota imap_quota"
}
protocol pop3 {
   pop3_client_workarounds = outlook-no-nuls oe-ns-eoh
   pop3_fast_size_lookups = yes
   pop3_lock_session = yes
}