Sometimes when I open the reactor control the client crashs.
http://pastebin.com/JyHFiifk

I have the same issue while holding the Resonant Energy cell from Thermal Expansion... know a fix?

Having the same problem while holding a TiCo tool

I have the same crash while holding an atomic disassembler from Mekanism.

Strange. Let me know if you can discover any reproduction steps. However, the stacktrace is pure vanilla, so I doubt this is BR.

Ticking memory connection indicates a problem with your ram, for example that you haven't allocated enough ram.

I guess ram is no problem.
allocated ram: 40%
latest log: http://pastebin.com/PMSXkJek
it never happens with empty hand.
held items while crashing: electromagnet, energy cell
maybe other things too

The second part of your latest log tells that the game tried to place the block that you have in your hand and in parallel it opens the GUI which causes a network packet problem.

Which mod(s) are the electromagnet and energy cell items from?

If they are the ones I am thinking off, then they are from EnderIO.

I'd assume TC = Tinkers' Construct

Of course

the magnet is from eio, energy cell from TE

New realization: It happens every time I open the control while holding an TC pickaxe.
right next to the pickaxe is a TC shovel (I mention it because of the right click feature of TC Tools):
Crashlog and latest log didnt change.

TC = Thaumcraft? This sounds like the tools might not be handling containerless inventories properly, but I'll try to take a look when I get a chance.