Why DNS query shows different result for some hours?

[Corman 4u]

I have had a domain name and set up with 2 DNS servers from a public service
provider.
Recently, I changed one of DNS servers with my IP, which has my own DNS on
LINUX.
So first DNS of the domain is my DNS server and the second is one of DNS
servers of the public service provider.
When I query the domain, sometimes it works for hours; dig command shows my
IP in Answer Section.
But sometimes it doesn't work; dig command shows my IP (my DNS) or the DNS
server of the business service alternatively.
This situation is repeating for several days; it works well for several
hours, but doesn't for the next several hours.
While DNS query shows different result at each query, outside clients get
Gateway Timeout with code=DNS_TIMEOUT.
Is there anybody who knows the reason and how to fix this problem?

Corman
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