Your far plane is 200000? Thats a huge figure. Some thing you could also try is tho scale your models, velocitys and far plane down. I normaly got 1 XNA unit equal 1 meter or any other real scale, and I never hade any proble with something flickering, as long as it realy is not at the same lavel.
And about the controls: You accelorate with 'W' and deccelorate with 'Space'. It appears to me that that could be a part of the controls of a differnt game, as 'S' normaly would be the deccelorate button, because its just closer the 'W'. ;)
A nother pile of bugs:
When you rotate you plane, it rotates not around its own axis, but around a differnt on. It appears to me that you actually controll the camera and the plane is sticked on the camere, but it should be reverse.
When you press space to stop the plane in the air, the following drop is not straigt down.
You can fly below the runway ^.^
When you hit the checkpoints in a different than showed by the arrow, you get a hit and the arrow points to the checkpoint that should be after that one you have just hit, but later you have to figure out all by your self which one you didn't hit.
As closer you get to earth, as smaler your shadow gets. It should be differnt.
Greg the Mad