KIAH is one of the most optimized sceneries out there, which can be easily seen by noticing how it's fps friendly compared to other airports that have a similar size.
The crashes on Xbox due to low memory are very well known issue but, if the scenery works with a default airplane, there's not much we can do if most of the (not expandable ) memory is already taken by the airplane itself. Do you have other large add-on airports, like KLAX or KFJK that don't have any issues with the PMDG ?
BTW, which Xbox model you have ?
