I recently updated my NVidia GeForce 6600 drivers; which made JK crash on startup.
But, if I did as you instructed here...
-Load JK, on startup, goto Setup-> Display-> Uncheck/Disable 3D Acceleration, changed res to 800x600, then loaded my Level.
It loads just fine (w/o crashing). The graphix looks like crap this way, but it loads. =)
Then as you instructed...
-while in that level, press escape, goto Setup menu again, Display, Check/Enable 3D acceleration, keep res at 800x600...then click ok and 'back to game'. Level still loaded, now graphix are perfect high-res again. =D
ps> I was in dire worry that I would not be able to continue editing my levels, but never be able to beta-test them again. Thanks to you, I can continue... =)
Now, if only NVidia and MS would fix the problem(s).