Hi there.
Well, 3 spots drop in rankings is surely not "quite dramatic drop". It can happen for many-many reasons. What I would recommend though is to check if the domain name you have purchased ever had (or still has) penalty or spam issues. Usually, if everything is clean, it wouldn't affect your rankings.
As for irreversibility - nothing is irreversable. However, if that domain was in fact the reason of ranking drop due to spam issues, then it might take longer to recover.
Yes, disabling would be the first thing to try if you think that was the reason. See how your rankings react, if there are any sudden changes. However, I do recommend to check other factors like your backlink profile changes, content cnahges, your competition SEO changes and so on.
Hope this helps.
