AVR-Chat

I am setting 2 outputs high (n milliseconds apart)

Problem is that when I look at them with my logic analyzer, the
distance between them going high isn't always what I've coded.

My configuration is:

Compiling with WinAVR 
ATMega8
External clock (8Mhz crystal)
My Fuses are set correctly

-------------------------------

I've modified the makefile to have:

F_CPU = 8000000



-------------------------------
C Source:


#include <avr\io.h>
#include <avr\delay.h>

int main(void) 
{	
	
	//set port c as all output
	DDRC = 0xff;
	
	PORTC = 0x20;
		
	_delay_ms(37);	
	
	PORTC |= 0x10;
	
	
}


At 30 ms the timing is correct.  Anything above or below 30ms, the
timing moves in the right direction but not in correct proportion.

Maybe it's my [PC based] logic analyzer.
Thanks for your suggestions.