Adivice needed! - Stuttery screen and no picture?

[Baker]

Ive now down loaded loads of planes and all is going well, but I now have found a problem with two planes.

1 - I loaded a Great Planes trainer and the file is GPTRH, but when loaded the plane isnt their? but you can still fly as if it were invisable?

2 - A couple of other planes like a bronco I loaded look great but as I fly them the screen is stuttering? but with the same scenery, all the other planes work great with no stuttering?

Any body got any ideas?
Thanks
Baker :p