Hello and thanks to anyone that can help me!

My furnace started acting up yesterday, I was wondering if its worth fixing, calling a repair person OR just buying a new one...
First it is a 15 year old furnace, it's a Goodman model GMP 100-4

what is happening is the fan cycles on and off even when the thermostat is off (I've replaced the thermo just in case and that didn't help) I don't mean it cycles every few minutes I mean it will turn on and off 3 times a minute and sometimes 3 times a second (completely random).
I have checked all the wires and connections seem fine, no burnt wires.
once in awhile it will actually run a regular cycle and heat up my home. Then it will go back into its odd cycling (don't think its to hot as its in my basement and its about 25F here).
sometimes it starts a regular cycle and runs for 15 + minutes, other times it starts a cycle and the blower comes on at the same time the exhaust fan comes on.
I have checked the temp of blower motor and its cold to the touch.

Just seeing if I can get any tips/info before I spend a bunch of money.

Thanks

Mack

It could be the fan limit control. The fan part controls the blower on most older furnaces. If you turn the blower to on manual at the thermostat does the blower come on and stay on?