Lennox Model G24M3-75A-4

When selecting heat, fan comes on with heat demand and ignitors start immediately clicking. This shouldn't happen. Ignitors should wait 45 seconds before clicking. It'll keep repeating until locked out.

When ignitors start clicking when blower starting, I'll move temp down and after turning system on/off moving temp up/down several times, make that numerous times over a two hour period, the system will finally work. Meaning, fan comes on and after a 45 second delay the ignitors click and the system works.

Furnace will work normally until termostat manually set to cool. Then problem starts over again when the need to return thermostat to heat.

Other symptoms a
It'll display solid diagnostic lights indicating control board failure after a few attempts, but not at first;
set fan from auto to ON and fan doesn't work, just makes a soft clicking noise, control board will indicate board failure;
Air Conditioner works everytime when placed to cool.

What/which parts do I change first?

What is causing ignitors to click at same time as blower starting?

Why doesn't my fan work when set to ON?

Please help.