My house was built in 1999. Now I am having trouble with my furnace, since las year in October every time it wants to run; the flame comes on & off about 4 to 6 times before it really works & sometimes it shuts off and won't work at all & it gets very cold in the house. I have paid twice to replace the flame sensor & it worked fine for awhile but this week started to do the same again; winter is coming & I'm so worried about this. What could it be?

Check for a bad ground where the spark box/controler is mounted to the furnace. Clean the sensor.

Check for a bad ground where the spark box/controler is mounted to the furnace. Clean the sensor.
I agree with hvac1000 you do have to have a good equipment ground on the control board and clean the flame sensor not replace it. You only have to replace it if the ceramic is broke and its going to ground.