Anyone ever had this type of game?

Once their allied, you're screwed. All you can really do is park transports next to one civ's planets to get them to declare war on you; If they declare war, their allies won't join the party.

In my very first game, on cakewalk, the Dominion of Korx allied with all nine major races, including myself.  Finally an assasination event occured and one of their alliances was broken.

I've had several games like this lately... Everyone gets chummy with each other and all conflict stops. Anyone working on a mod regarding AI behavior or something? What I really need is a cheat to set all players relationship with each other back to neutral or something sweet like that...

Another possibility is to spam some of the AI's with influence starbases. Either they declare war on you or you start flipping their planets. Or simply go for a tech victory.

In the cases where the same happened to me, it was often the assassination event that helped to break the standoff. Or the arrival of the jagged knife, leading to me taking over all jagged knife planets and increasing my influence significantly.

This is the reason why they need to change the diplomatic victory condition in GC3. Because no one is going to let the AI win by joining in on their alliance. Granted, I have seen the AI ally with a couple other AI's but never to this extent.

Yep happens to me every game I've played (Only played 2-3 games so-far tho)

This is why you want to keep bribing the AI to go to war, and get started on it early.  They won't be making alliances if they're too busy smashing each other's faces.