I am using TICTO to calculate the simulation time ,but each time when I simulate the variation is very large .Why it is happening ?how can I avoid this variation ?

Hi...I am really worried about the simulation time ..for instance ,I try to run a program for first 5 run the range is from 132 to 140s after a while the simulation time ranges from 123 to 126s and later it is in the range of 38 to 40.I don't know why there is this much variation?Can someone help me to get rid of this problem ?