Central Heat does not stay on?

My central heater ignition turns on, the gas is shot through the injectors then the flames turn on. The flames will stay on for 5 seconds & then the flames shut off. The blower kicks in I guess to blow the trapped gas out of the system. The system then tries again to turn the heat on & repeats the cycle a total of 3 times & after the 3rd time the system goes into lock out. When the heat does stay on it will heat the home. However, when the heat shuts off after heating the home it will go through the same trouble as stated above when it tries to come back on.
These are the things I have done: I've changed out the flint or ignition whatever it is called, I've removed the injectors where the gas comes out & I have used sand paper to clean them.
,
Any other suggestions??