What is your treasury at each turn?
I'm a bit rusty on the numbers, but I think if your treasury is above 100K you get decreased income from trade, and at 200K your tax income gets cut 50% and your trade income gets cut another 20% or something like that. (I used to have these numbers memorized, but it's been a while since I played)
This was built into the game as "graft" - to represent the greedy politicians of your empire padding their wallets and funding their own pet projects with all the excess money in the treasury.
This is and has been a hotly debated topic for a long time, with many players asking for a change to the system.