SEO question to the public

I have a gambling site which has not bad performances in Search engines. There is a problem with the subsection about casinos, the main keyword is "free online casino games for real money"
I'm not using geotargeting or anything like that. I bought some natural links and some blogposts, but I can not detect why does it loose positions from time to time 1 to 10 positions for each query per week. Can not detect what is the problem?
Maybe the problem is in the relevance of the article to "online casino games" ? There is no incomings in text
Is here anybody who can detect the errors in my strategy? Will be greatly indebted, if you have any questions - you are welcome.

Uhh, if you actually run a gambling website, isn't it natural for a ton of search engine fuckery? Like I wouldn't be surprised if alternate gambling websites were paying real money to somehow damage other sites results, over and above paying to improve their own results.

Anyways, fair warning. If any gambling websites links are dropped in this thread, this thread will be immediately deleted. EP is not part of the market to boost search engine results...