problem with large buffers in ram

Hi,
MCU: LPC2138
I am writing a code that takes data from serial port (interrupt driven) and stores it in queues.
Here's an overview:

//////////////
struct queue{
   char *buffer;
   int wp;
   in rp;
 };

char buffer1[6000];
char buffer2[1000];
char buffer3[600];
char uartBuffer[100];
struct queue myQueue;

void initQueue(char* bufferQueue, int size) {
    myQueue.buffer = bufferQueue;
    myQueue.wp = 0;
    myQueue.rp = 0;
}

void enqueue(char *data) {
  q = &myQueues;
  while(data[i] != '\0') {
        q->buffer[q->writePointer++] = data[i++];
        if (q->writePointer >= q->size)
                q->writePointer = 0;
        }
        q->buffer[q->writePointer++] = 0;
        if (q->writePointer >= q->size)
                q->writePointer = 0;
}

int main () {
  char str[50];
  UART0_init(9600);
  UART0_interrupt_init();
  initQueue(buffer3, 600);

  while(1) {
     sprintf(str, "wp: %d    rp: %d", myQueues.wp, myQueues.rp);
     UART0_sendstring(str);
  }
}

void UART0Isr() {
     UARTbuffer[i] = UART0_getbyte(); // get data until sequence completed
     if(/*sequence complete condition*/)
           enqueue(UARTbuffer);
}

Now, the problem is that the controller hangs after "enqueuing" 8 sequences(all sequences are same). and the number 8 decreases if i increase the size of sequence.

if i reduce the buffer3 size to 500 or less, code works perfectly fine.
for buffer3 size >= 500 ----> code hangs

if i interchange the declaration sequence of global variables like:

char buffer3[600];
char buffer1[6000];
char buffer2[1000];

it gets ok. Atleast for 1000 sequences.

if i disable all interrupts and use enqueue in main in a while loop, its runs fine.

void main() {
while(1) {
  enqueue(sequence);    //works fine here
}
}

Thanks for reading and advance thanks for helpful replies.

Regards,
Salman.