Elusive viagra type malware

Hello,
I’m having some issues with one of my websites.
Few months ago I noticed some weird viagra links in my website. Did some cleanup, scans, changed passwords, and forgot about it.
Since then the Google rank decreased, now the website doesn’t even show on Google.
Looking at the website from my computer and few others, everything looks fine, but when I check webmasters tools there are tons of “Not Found” errors.
All links are from this type:
https://www.dudukoren.com/translation-services/marketing-translation-service/www.d.umn.edu/tunnel/kirby/wp-content/themes/kirby/skins/?tada=generic-viagra-generic-meltabs-viagra&da=2

The text after “?tada=” is changing, but mostly it’s something to do with Viagra.

I ran a security check on Sucuri.com, and it found one error, in one page, which i can’t see from my browser or in the code.

I’m running out of ideas here. Please help!
What else can I do?

Thanks