I think this has been discussed somewhere in here, but I'm not sure where.  I have the same problem with some downloaded aircraft, you can learn to control this somewhat with a twist-handle joystick, but it still might be a problem. I've learned to live with it.  I have not noticed any pattern among various aircraft.  Try this: Military 04 page 12.  Download the XB-70.  For me, this particular aircraft has taken off straight and steady every time; I did one test flight out of Meigs once and didn't even have to touch the stick, it literally took itself off.  Maybe this will be an indicator as to whether your having a system problem or stick problem (stuff I don't know much about).  Other comments are certainly welcome here.  Good luck!

If you had to uninstall FS2004 you more than likely removed your FS9 cfg file and when you reinstalled FS it created a new one therefore losing all of the user settings you had. Make sure all of your keyboard assignments do what you expect them to as you lost any key assignments you might have had. Make sure your autorudder box is checked. This should help If you flew everything fine before the uninstall this would explain why you have lost control.

Go into your realism settings and make sure the sliders for P Factor, Torque and Gyro are set to the left. You might even want to check the box for Autorudder since you are flying with a keyboard.

Best solution however is to invest in a decent joystick even a low priced one for the time being. Shouldn't set you back much for a cheapie (I've seen some around the $20 range) and then you can save for a good one.