Char. LCD and 18F452 on 4 MHz working, but on 20 MHz not work.

Please help me.

Code:
define LCD_BITS 4
DEFINE LCD_DREG PORTD
DEFINE LCD_DBIT 4
DEFINE LCD_RSREG PORTC
DEFINE LCD_RSBIT 6
DEFINE LCD_EREG PORTC
DEFINE LCD_EBIT 7
DEFINE LCD_LINES 4
DEFINE LCD_COMMANDUS 2000
DEFINE LCD_DATAUS 50


LCDOUT $FE,1,"HELLO WORLD"