I have a 10-12year old 80000 BTU Rheem gas Furnace, and it is cycling every 2-3 min.
The fan comes on, the igniter heats up and the gas goes on and burns, and then 2-3 min later it shuts off and 1-2 min after it start the same cycle over again.

Now I have checked and cleaned the flame sensor and I see the tip of it glow red when the furnace goes through the cycle. Can the flame sensor be tested or is it a trial and error method with a new part?
I have read that it could also be the ignition module which could heat up and kick out when there are as many years as my furnace has. Can this be tested as well?

The thermostat is digital and was installed about 2 months ago. This problem has occurred in the last week.
I installed the old mechanical one that was there prior to the digital with a new piece of wire direct to the terminal board with the other leads removed. Even with the thermostat system isolated, the furnace still cycles the same.
Does someone have any other suggestions?

If you have the instructions for the new digital thermostat look up to see what it says about setting the "Heat Anticipator" or anything about setting the amperage on the thermostat. I am not saying this is the problem but it does sound like the symptoms.

Does the main blower ever come on or just the inducer? If not, leave the heat anticipator alone, and check the blower. Often as not the relay on the circuit board is bad. They are hard to check out and usually it means replacing the expensive circuit board.

Try setting the fan to on and running the furnace. If that works, it definitely the blower controls. ON uses a different winding and relay than auto in heat.