Hi Black Cat,
sorry to hear that you are frustrated. The reason why the AI turns on you is the MEE-Switch (Mega Evil Empire). You have out-scored the AI by a huge percentage and now the all turn to get you down to size. If you don't like this setting, go to your Data-Folder in SE4 (if you use a mod, go to the data folder in the mod), open the settings.txt and search for "mega". Set "AI Uses Mega Evil Empire" to "False". This should stop the erratic behaviour of the AI. By the way, if you out-score the AI by such a percentage I think you are way better then it might look to you. I'd say you could crush the AI if you keep it on.
It might seem to you that the AI knows where you have fleets and where you are weak but in reality the Ai doesn't. The AI in SE4 does not cheat and has no info that you don't have also. Trust me, I have looked over the AI shoulder for several dozens to hundreds of hours. Another tip, I'd recommend to set the AI difficulty to high. This won't give the Ai any bonus in form of resources or research points but it will enable all the AI ministers. If you set this to medium, you might cripple the AI quite a lot.