Yes, I have the exact same issue. Just made the crucible furnace, and when I open the GUI, the game lags like h***. I tried putting a piece of ore in it, and the game crashed...
I use the FTB-launcher and I get this issue.
Here is the crashlog:
http://pastebin.com/GJDLrZVQ
I have found that this happens if you open the crucible by clicking on the bottom block of the multiblock. Clicking on the top block of the multiblock seems to not give me this issue.