RPM fluctuating on PIC18 (PIC18F458)

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sbit LCD_RS at RC2_bit;
sbit LCD_EN at RC3_bit;
sbit LCD_D4 at RC4_bit;
sbit LCD_D5 at RC5_bit;
sbit LCD_D6 at RC6_bit;
sbit LCD_D7 at RC7_bit;
sbit LCD_RS_Direction at TRISC2_bit;
sbit LCD_EN_Direction at TRISC3_bit;
sbit LCD_D4_Direction at TRISC4_bit;
sbit LCD_D5_Direction at TRISC5_bit;
sbit LCD_D6_Direction at TRISC6_bit;
sbit LCD_D7_Direction at TRISC7_bit;
 
// Define Messages
char msg2[] = "RPM=";
char *rpm = "0000";
int RPM1;
int RPM2;
long Calc_RPM;
 
// Display RPM
void Display_RPM(unsigned long rpm2write) {
  rpm[0] = (rpm2write/1000)%10 + 48;
  rpm[1] = (rpm2write/100)%10 + 48;
  rpm[2] = (rpm2write/10)%10 + 48;
  rpm[3] = rpm2write%10 + 48;
Lcd_Out(1, 5, rpm);
}
// Main Program
void main() {
 CMCON = 0x07;    // Disable Comparators
 ADCON0 = 0x00;   // All AN to Digital Input
 ADCON1 = 0x00;
 TRISC = 0x00;     // PORTC O/P
 TRISA = 0b01010000; // RA4/T0CKI input, RA6 is input for Ext OSC
 
   T0CON = 0b01111000; // Prescaler (1:1)
//                 0-------   TMR0ON off
//                 -1------   T08BIT is set for 8 bit counter
//                 --1-----   T0CS is set T0CKI pin
//                 ---1----   TOSE set increment hi-low
//                 ----1---   PSA Prescaler not assigned
//                 -----000
 
// Initialize LCD display
 Lcd_Init();                 // Initialize LCD
 Lcd_Cmd(_LCD_CLEAR);        // CLEAR display
 Lcd_Cmd(_LCD_CURSOR_OFF);   // Cursor off
 Lcd_Out(2,1,msg2);
 
do {
  T0CON.TMR0ON = 1; // On the timer
  TMR0L=0;
  Delay_ms(1000);  // Delay 1 Sec
 //-- RPM Calculation = (60 x Frequency)/PPR --//
  Calc_RPM = (60*TMR0L)/4;
  Display_RPM(Calc_RPM);
  } while(1);  // Infinite loop
}