Intermittent 503 errors reported by Uptime Monitor

I have a multinetwork website and the Uptime Monitor is spamming me with 503 Service Unavailable down reports. The website is always up!

I've checked the access and error logs and there are literally seconds apart calls from other sources that respond with code 200 but the Uptime call reports sometime 503 and sometime 200.

I am not sure if related, but the wp_debug reported multiple PHP notices related to WPMU plugins:

PHP Notice: Undefined index: HTTP_USER_AGENT in ../wp-content/plugins/wp-defender/app/module/ip-lockout/controller/main.php on line 477

Here are the access logs that reports the 503 , and the previous/next calls that return 200:

xxx.xx.xxx.xxx - - [25/Jan/2018:02:46:36 -0800] "POST /wp-cron.php?doing_wp_cron=xxx.052xxx HTTP/1.1" 200 3408 ".../wp-cron.php?doing_wp_cron=xxx.052xxx" "WordPress/x.x.x; **DOMAINNAME**"

xx.xxx.xx.xx - - [25/Jan/2018:02:47:00 -0800] "HEAD / HTTP/1.1" 503 3523 "**DOMAINNAME**" "WPMUDEV Uptime Monitor 4.0 (https://premium.wpmudev.org)"

xx.xxx.xx.xx - - [25/Jan/2018:02:48:55 -0800] "HEAD / HTTP/1.1" 503 3523 "**DOMAINNAME**" "WPMUDEV Uptime Monitor 4.0 (https://premium.wpmudev.org)"

xxx.xx.xxx.xxx - - [25/Jan/2018:02:50:40 -0800] "POST /?wordfence_sync=1516xx.xx9 HTTP/1.1" 200 3417 "**DOMAINNAME**/?wordfence_sync=15xxx.xx89" "WordPress/x.x.x; **DOMAINNAME**"