Why is java.util.Random slowing later lines of code?

I am trying to test the runtimes of various disk accessing patterns and have noticed that any time I use a randomly generated data set the execution takes roughly 2.5 times as long as usual, for both a cached and noncached access method. The non-cache, straight to disk, accesses have the same execution times for all of the other access patterns. The random accesses are the only ones that differ by a significant amount.

Is there a reason that the //testing stuff here section of my code runs slower when using accessPattern[i] = rand.nextInt(1000); rather than accessPattern[i] = i * 2;? I pass the elements of accessPattern[] one at a time to another function in the testing section, but I do not see why this would be affected by initializing the array using Random.

private void testRandomAccesses( ){
    SysLib.cout( "Random Access Test\n" );
    Random rand = new Random();
    int accessPattern[] = new int[400];
    for ( int i = 0; i < 400; ++i )
        accessPattern[i] = rand.nextInt(1000); //slow execTime
      //accessPattern[i] = i * 2; //fast execTime


    startTime = new Date().getTime();
    //testing stuff here
    for ( int i = 0; i < 400; ++i )
        SysLib.rawwrite( accessPattern[i], testBlock ); 
    endTime = new Date().getTime();
    execTime = endTime - startTime;
}