Jealous sometimes and not others?

So when it comes to my relationship, I wouldn't say I am extremely jealous. I usually only get jealous in two circumstances... 1) my girlfriend says and compliments other men by calling them attractive or charming, or 2) she is talking to another specific guy. Now this is the problem wwith#2..... its only certain people. She will talk to someone or hangout with them alone and I will be completely fine with it, but other people I will just get this churning in my gut of jealousy. Im not sure what the pattern is... these people are usually just randomly picked. It isn't a matter of how well I know them, or anything like that. Has anyone else has this happen and what's a way to help it?