When computing load increases, the CPU gets hotter. The symptoms you
have described are the result of a overheated CPU. The next time you
restart check the CPU Temps in BIOS.
It is also a possiblility that one of hard disks have a loose
connection. Refix all connections - data and power.
Blue screens are also a result of corrupt RAM. Use memtest (google for
download location) to ascertain RAM status. REfix RAM modules.

The reasons all this is happening now is because the PC is growing old
and components are approaching their "end". Also the removal of the
hard disk could have caused loose contacts.