Simultaneous running problem

hi.

i have a serial fortran code.when i run alone, it converged in 20 min and final answer is 17

but when i Simultaneous  run it in more than one vs it converged in different time and different answer!

i work with vs2012 and intel xe.

Here's to say that with 4 Simultaneous run, about 50 percent of my cpu and ram remains free 

whats the problem?

thanks.