bug Tekkit Main crash with Thermal Expansion

[SonicCreeper7]

In my current Tekkit Main world (version 1.2.9f which was recommended at the time) I have some thermal expansion Leadstone Fluxducts. They work perfectly fine, no problems, however, if I unload the chunks which the ducts are in (e.g. go to the Nether) and come back again, they look as though they are not there. You cannot see them with your crosshair, cannot make them disappear, but they still work perfectly fine. You can get stuck in them, but if you place a machine that uses power next to where you think they are, the game crashes. Any help?