If this is happening within the weldment assembly environment, you might try actually double clicking and accessing the Machining group and then editing your sketch from there. If you try to edit the sketch without first clicking on the Machining group then you may experience this problem. Trust me, it works if you do what I say. Still not sure what causes it.
IV2012
Windows XP SP3 32-bit
Intel Core 2 Duo 6400 @ 2.13 GHz
Nvidia Quadro FX 3450/4000 SDI 256MB Vram
2 GB Ram
160GB HDD