Intermitant crashes on flare

I've been testing out a custom runway i've made and have a misterious problem.Here's how the runway is constructed:Invisible FS2k2 runway with 2 texture mapped polys layered over to show the asphalt. Surrounding land is flattened used two flatten areas at 12ft MSL with a gap between. In the gap is another flatten area at -8 feet MSL which creates a trench running under the runway. Unfortunately, since i added the texpolys, the land stretches up to meet one where it crosses the ditch, but i don't mind this too much.I then tested this to make sure the planes don't fall through or anything. I did a circuit in the Dreemfleet Archer, FSD Seneca and LAGO twotter. No problems. Then i did an approach in the Project Airbus A320 and just before i flared (at the point where the ditch goes under the runway) MS says that i have crashed. Yet i'm still about 50ft AGL! I tried again and it worked fine. Then i tried a 3rd time and crashed again!I can't work this out but i'm sure it's something to do with my ditch. Yet why is it only on one plane and only intermitant?ARRGH!