1) Deadzone means how far you can move your joystick, without anything happening. If your joystick is properly calibrated and you still can't fly straight when you let it go, increase the deadzone.
2) If the exe you use has a d behind the number, switch to one with r instead (like "fs2_open_3_6_13r_INF_SSE.exe"). d are the Debug builds and only for developers and troubleshooting. r -> release are for regular playing.
If you use an r exe already, check the "Dev Tools" tab in the launcher, maybe you got a few flags activated there.
If neither of those is the case, I'm out of ideas.
