Why is my website redirecting to a spam site?

I have a website, that is every few months sending users to a spam site (like virus) or to a page where it says domain not found, buy it, etc etc.

Thus far, the only solution to this problem has been to resolve the DNS cache on my machine, which then replicates to other servers and users are then able to access the site again. I can also resolve by changing DNS servers locally or at the network settings, (google, opendns, etc) this usually fixes the problem as well.

But then it happens again at some point, might be 2 months, might be 6, who knows. I contacted the hosting provider and they told me to clear the DNS, which as explained above, works only for a while.

The issue I suppose started happening a few years back, when we were experimenting with a wordpress site in a separate folder. Since then, I have deleted all database entries, folders and files that contained this old WP site. Also, this started occurring AFTER I switched my domain from my old host company to my current host (to get everything under 1 roof).

Hoping someone may help me understand if this is an issue with my ISP/Host, DNS Hack, malicious code, etc? in my site I havent found yet?