If you Google "503 error" you'll find articles such as "Lifewire: 503 Service Unavailable Explained"

Do your server logs show anything out of the ordinary during the periods of these outages? What are the system resources looking like when these messages are being encountered?

I am a Web Development Contractor, I do not work for UBBCentral. I have provided free User to User Support since the beginning of these support forums.
Do you need Forum Install or Upgrade Services?
Forums: A Gardeners Forum, Scouters World
UBB.threads: UBBWiki, UBB Styles, UBB.Sitemaps
Longtime Supporter & Resident Post-A-Holic
VNC Web Services: Code Modifications, Upgrades, Styling, Coding Services, Disaster Recovery, and more!