Heap corruption detected at 01670000 While Using CString

The Following is my function:

CString CMem::GetALLData(UINT uStartStep, CString cRequests)
{
    CString Package = "",sSteps, Buffer = "";
    cs_Mem.Lock();
    try{
        int PLen = cRequests.GetLength(), Size = 0, Steps;

        if( Valid() && pMemStatus != NULL && pMediaBuff != NULL && PLen !=
0){
            for( int i = 0; i < PLen * 8; i++)
            {
                // if the request part is not in the list
                if( uStartStep + i > CurrentStep && uStartStep + i < CurrentStep +
MemSize)
                {
                    Steps = uStartStep + i - CurrentStep;
                    //if( pMemRSTimes[Steps] <= 0)
                        //TRACE("^^^^^^^^^^^^^^^^^^^^ Times Useout!!!
^^^^^^^^^^^^^^^^^^\n");
                    if( ( cRequests[ i / 8] & ( 1 << ( 7 - i%8))) > 0 &&
pMemStatus[Steps] == 1/* && pMemRSTimes[Steps] > 0*/)
                    {
    // if( Size > MaxRequestSize)
    // break;
                        pMemRSTimes[Steps] --;
                        sSteps.Format( "%10d", uStartStep + i);
                        Buffer = CString( pMediaBuff + 1000 * Steps, 1000);
                        Package += sSteps;
                        Package += Buffer;
                        Size ++;
                    }
                }
            }
    // if( Size < MinRequestSize)
    // Package = "";
        }
    }
    catch(CException* e)
    {
        Package = "";
        e->Delete();
    }
    cs_Mem.Unlock();
    return Package;
}

It stop with the error "Heap corruption" at "Package += Buffer "about
2-3 hours after I start the program.

Any Advice is OK,
thank a lot