Problem with reactor dissappearing

[DiamondXXPickAxe]

I have a recurring problem with my world. I opened my world and I replaced my lava generation system with a big reactor in my cave base, but when I logged back, the reactor was completely gone, the lava system was still there, and I have the same amount of items and block placed before I the last time I logged in. So I build the reactor again thinking it was a short term problem and logged out but the lava generator was still there. Also when I logged in, it lagged, and I went straight through the floor.'

 

Tekkit (Java 3.5 GBs and 64 bit system) was getting very laggy and was making my computer lagging so I was restarting it constantly.

 

I'm just asking 'What the heck!!!' and to please help!