Home heating unit Weil Mclain steam gas furnace model #EG-50-PI made in 1989

I have a home heating unit Weil Mclain steam gas furnace model #EG-50-PI made in 1989. Every time when it run for over 4 minutes, there would be a low "tick" sound and the flame would go off, 2 second later the flame come back on for 8-10 second then the "tick" sound heard and the flame went off again. This would repeat until the gas shut off valve shut off the gas. Do you know what is the cause?
Any response and advise will be greatly appreciated! Thanks!

I had the same issue with my heater and that was because it was too old, mine was 1990 made. I recently bought a boiler for Heating Oil in Belfast at a reasonable price and it runs perfect and the company I buy oil from is very nice and the services are superb.

http://www.valueoils.com/