Atmel's AT91SAM7X USB write sometimes takes long time

//*----------------------------------------------------------------------------
//* \fn    AT91F_CDC_Write
//* \brief Send through endpoint 2
//*----------------------------------------------------------------------------
static uint AT91F_UDP_Write( AT91PS_CDC pCdc, const char *pData, uint length )
{
    AT91PS_UDP pUdp = pCdc->pUdp;
    uint cpt = 0;

    // Send the first packet
    cpt = MIN( length, AT91C_EP_IN_SIZE );
    length -= cpt;
    while( cpt-- ) pUdp->UDP_FDR[AT91C_EP_IN] = *pData++;
    pUdp->UDP_CSR[AT91C_EP_IN] |= AT91C_UDP_TXPKTRDY;

    while( length )
    {
        // Fill the second bank
        cpt = MIN( length, AT91C_EP_IN_SIZE );
        length -= cpt;
        while( cpt-- ) pUdp->UDP_FDR[AT91C_EP_IN] = *pData++;
        // Wait for the the first bank to be sent

        while( !( pUdp->UDP_CSR[AT91C_EP_IN] & AT91C_UDP_TXCOMP ) )      // ISSUE: sometimes, it takes a long time to complete transmission
           if( !AT91F_UDP_IsConfigured( pCdc ) ) return length;

        pUdp->UDP_CSR[AT91C_EP_IN] &= ~( AT91C_UDP_TXCOMP );

        while( pUdp->UDP_CSR[AT91C_EP_IN] & AT91C_UDP_TXCOMP );

        pUdp->UDP_CSR[AT91C_EP_IN] |= AT91C_UDP_TXPKTRDY;
    }
    // Wait for the end of transfer

    while( !( pUdp->UDP_CSR[AT91C_EP_IN] & AT91C_UDP_TXCOMP ) )      // ISSUE: sometimes, it takes a long time to complete transmission
       if( !AT91F_UDP_IsConfigured( pCdc ) ) return length;

    pUdp->UDP_CSR[AT91C_EP_IN] &= ~( AT91C_UDP_TXCOMP );

    while( pUdp->UDP_CSR[AT91C_EP_IN] & AT91C_UDP_TXCOMP );

    return length;
}