hi,

Prahlad, I looking at code and see some not good coding. Let me cut next part of code:

	if (onfire && SYSTIME>firedly && firecount  && fwbit)
	{
		FIREPORT=!FIREPORT;	//Put the fire pulse	
		firecount--;		//Reduce remining pulse counter.
	}
	else
		FIREPORT=1;		//Make fire pin 1 to keep transistor off.
	fwbit=!fwbit;			//fwbit to make firing pulse 200us
	ioport=OPSTATUS;
}

First of all, I need to say that you toggle fwbit at each interrupt even if firing is not required. As result, you never be sure that at time when zero cross has been detected this bit is at state 1. As result, first firing pulse delay is not fixed and varies as (firedly + [0 or 100us]). I may suggest you to set this bit manually inside both cases:
if (IE0) {... fwbit=1 ...}
if (SYSTIME>99) {... fwbit=1 ...}
...but read below.

Secondly, your comment says:
fwbit=!fwbit; //fwbit to make firing pulse 200us

If you mean low level pulse duration time then it does not. The duration is 100us because next line:

else FIREPORT=1;

Let explain:

	if (IE0)		//If zero cross detected.
	{
		SYSTIME=0;	//increase systimer.
		cycles++;
		IE0=0;		//Clear zero cross flag.
		firecount=4;	//set fire couter.
	}

1. zero cross or SYSTIME detects that it needs firing. It loads number of pulses (firecount) and resets SYSTIME to count delay before actual firing (SYSTIME>firedly).
2. SYSTIME>firedly happens. Assume that fwbit is set at this time. So condition FIREPORT=!FIREPORT; makes first pulse (low level) and decreases firecount. fwbit inverts at the end of ISR.
3. Next interrupt (100us) comes on and so condition if (onfire && SYSTIME>firedly && firecount && fwbit) fails. Instead program does else FIREPORT=1;. By this, output comes to high level. As result, low level pulse duration time is 100us.

What is about next code:

	SYSTIME++;		//increament the SYSTIME variable used for fire timing
	if (IE0)		//If zero cross detected.
	{
		SYSTIME=0;	//increase systimer.
		cycles++;
		IE0=0;		//Clear zero cross flag.
		firecount=4;	//set fire couter.
	}
	if (SYSTIME>99)		//If SYSTIME exceeding 100 it is a zero cross.
	{
		SYSTIME=0;	//reset system timer.
		firecount=4;	//set fire counter.
	}
	if (onfire && SYSTIME>firedly && firecount)
	{
		FIREPORT=(firecount & 1);	//Put the fire pulse	
		firecount--;	//Reduce remining pulse counter.
	}
	else
		FIREPORT=1;	//Make fire pin 1 to keep transistor off.
	ioport=OPSTATUS;
}

Here I have fwbit deleted (its function is done with bit 0 of firecount).

Regards,
Oleg