I have always had sporadic crashing to desktops on Isengard instances, but they weren't common so I tolerated them, but ever since U7 every Isengard instance I have entered I have crashed upon loading. I have to restart the game to get into my instance which takes a long time. I have windows 7 and every other instance runs fine, Turbine please fix these buggy isens.
I don't do Isen myself - mostly needs group, so it's not high on my to-do list. However, I have seen another forum poster complaining about a similar symptom. In their case, it turns out to be an incompatibility with their current sound set up. You might want to try playing with the sound off for a while, as an experiment, to see if stability improves.
as mentioned above muting ingame sounds during transitions seems to be a temporary fix for many. Alternately you may also try updating your video card drivers, I had crash to desktop issues tho not on transitions and updating my drivers to latest beta drivers fixed my issues completely, YMMV.