Windows Crashing on 12GB (3x4GB 1600) RAM Dell Vostro 430

I have a DELL VOSTRO 430 and i just upgraded it from 8GB ram to 12GB ram. i was using 1600MHz DDr3 ram previously but after i upgraded to 12gb ram the pc booted  the very first time ok with message of memory increased but next day the ram decreased to 4GB while the BIOS and Windows were detecting all 12GB ram then the pc started to crash and when i restarted it the pc directly shut down from windows 10 boot logo. I had to buy new pair of 4GB 1333MHz DDR3 ram and it didnot make that issue again. Here are the specs my vostro 430 has

CPU: i7-860

RAM: 12GB

GPU: GTX 460

I just want to ask does RAM speeds make this much difference.
I know this pc is really old more than 10 yrs old but i use it as a stream and office pc and it does great work for it.