What power are the displacements? I think (don't quote me) that the higher the power of the displacement, the more your resources are used to handle physics collision. This would explain why your game crashes when the sentry gun explodes--it can't handle it.
I would suggest a few tests:
play demoman and shoot some pipe bombs and stickies on the displacement surfaces. If the game crashes in the same way, reduce the power on your displacements. You shouldn't have to have anything above power 4, even though it does look better.
If you simply can't reduce the power, make sure there is nodraw behind the displacement and then select your displacement and check the 'no physics collision' on the face edit options on the displacements tab. This probably is not the best solution, since now the displacement is non-solid to a player, but it should work.
Otherwise, you're having a problem completely unrelated to what I think is happening in your map. Good luck!
EDIT: Can you say exactly what kind of crash you're having? What does hl2.exe tell you when it crashes, or does it at all? That might help determine the problem.