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This is a bad blower switch. It activates with heat for on/off temperature.
The switch is kicking on, then cooling a bit and turning off, then heats up and turns on again.
Yours will be similiar to the one at this link.
Except your is not adjustable and the one pictured is. The adjustable lets you pick how fast you want it to kick on and then how long to run to cool the furnace. see here