AccessViolationException while copying data from C++ to C#

1. C++ project, which is built into DLL with "/clr", and has native C++ code and Visual C++ to call native C++ from C#.
2. C# project, which is drawing windows and control multithreading.

The idea is simple - native C++ program gets initial data, makes some calculations (about several seconds) and output results array with a lot of data. I need to make a lot of these calculations for various initial data. So I made it like this:

• C# parse file with initial data

• C# launches a parallel.for cycle:
  --------------------------------------------------------------------------------
  parallelOptions.MaxDegreeOfParallelism = 8; // for 100% load on 8 cores
  Parallel.For(0, data.NUM_TRAJECTORIES, parallelOptions, i => {
  // call Visual C++ to call native C++
  });
  --------------------------------------------------------------------------------

• In unsafe ('cause I need pointers in C# to fill C# array from C++) void function C# creates an array for results:
  data.dataFromCPP[i] = new double[result.GetSize(), 8];        // 8 - is the number of output parameters. result.GetSize() - returns number of rows for current finished calculation

• Using pointer to array, C++ fills C# array with output data:
  fixed (double* p = data.dataFromCPP[i])
  {
          result.CopyResults(p);
  }
  --------------------------------------------------------------------------------
  And here's the function from native C++:
  void cSolver::copyResults(double *externalArray)
  {
          for(int i = 0 ; i < resultsVector.size(); i++)
          {
                  externalArray[i*8 + 0] = resultsVector[i].lat;
                  externalArray[i*8 + 1] = resultsVector[i].lon;
                  externalArray[i*8 + 2] = resultsVector[i].altitude;
                  externalArray[i*8 + 3] = resultsVector[i].tendency;
                  externalArray[i*8 + 4] = resultsVector[i].time;
                  externalArray[i*8 + 5] = resultsVector[i].Vx;
                  externalArray[i*8 + 6] = resultsVector[i].Vy;
                  externalArray[i*8 + 7] = resultsVector[i].Vz;
          }
  }

The Problem:

The problem is in AccessViolationException while copying data from C++ to C#, last time it occurred on this line

externalArray[i*8 + 3] = resultsVector[i].tendency;

It appears rarely - I've launched program 4 times, for the first 3 times it done everything excellent, and on the fourth it stopped with this exception. But that's really bad, 'cause for 3200 calculations - it takes about 20 minutes to calculate them all. And when it is stopped on 3197' trajectory - it is reeeealy terrible...

So, can anyone help me with this problem? What am I doing wrong, and what should I do to avoid this exception.

Best regards,
Nikita Petrov.