Array overruns or something going on

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C++

Linux

GCC

I have defined a pretty simple class that I'd like to manage an array of blocks of data, arrays of 256 bytes each, with a max number of 256 blocks. I'm using a std list to hold the arrays. For testing I'm creating a an array of same, memsetting it to 0, loading up the array with uint8_t data, inserting that block into the list, and then memsetting that block again with 0, loading it with new data, and inserting that. My results are like this:

uint8_t ablock[MAX];
string stuff = "Now is the time for all good men";
memcpy(&ablock[0], reinterpret_cast<const uint8_t*>(&stuff[0]), stuff.length());

I then invoke a class member to insert the data block into the list, a private member of the class. Appears to work fine. I then repeat the process re-using the data block after memsetting is again to 0. The data blocks are of type void, the intention is to make the class data types agnostic, as much as I can.

First insertion is fine. I can pull the block out and read it, converting from the void type to uint8_t and finally putting the data into a std string with the proper conversions, no problem.

The problem is when I insert a 2nd string of data into the class, even though I memset the buffer to 0 I'm getting what appears to be an offset situation.
2nd insert and pull of the data:

Now is tTo come to the aid of the party.
To come to the aid of the party.

Here's the relevant part of the code:

class MemMgr
{
    unsigned _idx;
    list<void*> _blocks;
    void* _mempool[MAX];

public:
    MemMgr(void);
    ~MemMgr(void) { }
    void* GetBlock(unsigned);

    void InsertBlock(void*);
    unsigned GetSize(void) { return _blocks.size(); }
    void clear(void) { _blocks.clear(); }

    void DisplayAll(void);
};

void
MemMgr::InsertBlock(void* block)
{
    if(sizeof(block) > MAX)
        throw runtime_error("Blocksize too big");

    if(_blocks.size() > MAX)
        throw runtime_error("Out of room");

    memcpy(&_mempool[_idx], block, MAX);
    _blocks.push_back(&_mempool[_idx++]);
}

The MemMgr constructor initializes the _idx member to 0;

I can post more code can be posted if necessary.

What I have tried:

Tracing through with my IDE (Qt Creator) but all the deugger shows are addresses, I can't see the contents of each block in the list.