These are a few of the weird landing characteristics that my Project AI planes do.

1) The aircraft landing two at a time.  ???
2) Aircraft doing go arounds when there are no aircraft near the runway. ???
3) Aircraft that attempt a landing abort, pull up sharply, then tailslide back into the ground and crash.
4) Aircraft that sit on the taxiway for 15 min.  Then "disapear."
5) Aircraft that land "hard".
6) Aircraft that taxi onto the grass.
7) Aircraft that stop when turning onto the taxiway, and stay there for 10 minutes, preventing other aircraft from landing or taking off. ???

I ran AFCAD and did not find any problems with broken links. Can anyone give me some advice on what could be happening and how to fix it? This is for default scenery in FS2002. I have ttools but it does not seem to help.

Any advice would be greatly apreciated.

the 2 AC landing at once is multiple flight plans for the same plane.

Check the flightplan text.

For the sitting plane, check the departure time and FL, if they are 00:00:00 or FL000 the plane can't depart.