I agree with the other comment. Look into the actual logs of the services. If they send a 503, they should be able to provide an explanation.
If you're asking if your ISP is alright... You can monitor that. Monitor if DNS is working, monitor if a ping to some server has hiccups.
And then do it methological. Is it just completely random services? Then it's likely that your monitoring has connectivity issues. Or is there some structure to what you're seeing? Do the issues all concern the same server? Or location? Or protocol? Then it's maybe that. Or it's a bit more complicated but they share a common thing, software or infrastructure element.
Edit: Alright, I didn't notice this was all concerning your same homeserver.... Maybe set up some local monitoring? See if it's different from the perspective of the computer itself, or just if viewed from the internet? You can also monitor some performance parameters: Is there enough free RAM, is the CPU busy, are you close to maximum upload bandwidth, is the I/O too much... But I suppose the main question is: Is it a network issue? And if yes, where and what kind.
If you're using Cloudflare or some other tunneling solution, that could also be the issue.