When it is time to heat the fan on the furnace comes on and the furnace lights for 2 or 3 seconds then goes out. About a minute later it will light again for 2 or 3 seconds then go out. This will happen for about 7 or 8 minutes until finally the furnace will stay lit. The fan will then come on and heat the house. The small blower? fan will stay running the whole time that it is trying to light but the big fan will only come on when it is up to temp. What is wrong with my furnace????