sevice status in WHM shows:
Server Load 0.01 (8 cpus)
Memory Used 13.3 %

but I get alerts from The Planet Monitoring System saying HTTP is CRITICAL
what can I do?

thanks

Verify if it's a monitoring or a server glitch: either see if you or an employee can be watching the server and verify right when the orbit ip alert gives you a critical warning, or else setup an additional external monitoring to check. I have seen the ip alert glitch on more than one occasion though it's usually pretty good. Or open a ticket and ask a tech to investigate next time it goes off. There was a network issue today if you look at the announcements forum which could well have caused the ip alert to go off unrelated to your actual server.