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run time variability

I have a question about run time variablity in Mathematica.

I am running the same program with different inputs, counting the  
computational time for each input.
I get highly variable run times with different inputs and am  
wondering if there is something in the computing envirionment that is  
the cause and not just the complexity of the input.

I am counting paths through networks of various sizes, consisting of  
between 35-70 nodes and 130-300 edges (70, 300 the largest; 35, 130  
the smallest).  run times vary from 10 minutes to 28 hours, with 27  
and 36 minutes for the two data points in the middle.  there does not  
seem to be any pattern

timing and results are below:
edges		nodes		time(sec)	#paths			sec/path
158			49			627			493,180			0.0013
131			36			1662		2,578,174		0.0006
167			48			2212		3,601,321		0.0006
304			73			101106		2,685,255		0.0377

the code is inefficient and my problem computationally intensive, but  
my key concern here is the wide variablity in performance.  Could it  
be the specific topologies of the cases that causes the variablity?


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