PIC18F4431 Configuration

Hello,

I'm trying to simply configure a PIC18F4431 to generate an interruption when the Timer0 overflows.
When this interruption occurs, I decide to switch on or off a led.
The oscillator is a 20Mhz, and I configure the Timer0 in 16 bits mode.
So normally, the interruption has to come every (65535*4/20000000) according to the datasheet

My problem is that in my program, the interruption does not come every
(65535*4/20000000) but every 4 or 5 seconds.
And when I "scope" the osc1 and osc2 pins, I don't have a sinusoidal signal but a signal sticked to 5V.

Somebody could help me ?
Thanks

Here is my code :
The flags are defined in another C file


//===========================================================================================================================================
//; assembler/linker/chip options
// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
// nolist
#include <P18F4431.h>
#pragma config OSC = HS



//===========================================================================================================================================
// Constantes definition
// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

#include "application.h"
#include <stdio.h>
#include <P18F4431.h>

#define RA0 OUT
#define RA1 OUT
#define RA2 OUT
#define RA3 OUT
#define RA4 OUT
#define RA5 OUT
#define RA6 IN
#define RA7 IN

#define RB0 OUT
#define RB1 OUT
#define RB2 OUT
#define RB3 OUT
#define RB4 OUT
#define RB5 OUT
#define RB6 IN
#define RB7 IN

#define RC0 OUT
#define RC1 OUT
#define RC2 OUT
#define RC3 OUT
#define RC4 OUT
#define RC5 OUT
#define RC6 IN
#define RC7 IN

#define RE0 OUT
#define RE1 IN
#define RE2 IN
#define RE3 IN

#pragma interrupt Low_Irq
void Low_Irq(void)
{
unsigned int i;mybyte b;
//- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Irq_Timer0_Match:
if (INTCONbits.TMR0IF){
INTCONbits.TMR0IF=0;
FLAG_2500US=1;}}


void main()
{
INTCONbits.GIE=1;
INTCONbits.PEIE=1;
INTCONbits.TMR0IE=1;
OpenTimer0(T0_16BIT & T0_SOURCE_INT & T0_PS_1_1);


ANSEL0=0;
ANSEL1=0;

TRISA=RA7<<7 | RA6<<6 | RA5<<5 | RA4<<4 | RA3<<3 | RA2<<2 | RA1<<1 | RA0;
TRISB=RB7<<7 | RB6<<6 | RB5<<5 | RB4<<4 | RB3<<3 | RB2<<2 | RB1<<1 | RB0;
TRISC=RC7<<7 | RC6<<6 | RC5<<5 | RC4<<4 | RC3<<3 | RC2<<2 | RC1<<1 | RC0;
TRISE=RE3<<3 | RE2<<2 | RE1<<1 | RE0;





for(;{

if (FLAG_2500US) {
FLAG_2500US=0;
Led_Cpu=!Led_Cpu;
continue; }

continue;}


}