Furnace runs for about 5-minutes only and shuts off
The limit switch should always be at 0 ohms. Since it is bouncing around it sounds like a bad switch. Check the temperature at the outlet of the furnace and at the inlet of the furnace. Subtract the two to get you the delta T. Look on the name plate for the correct delta T for your particular furnace. It will always be a range such as 30-60 degrees. If you are over the max number you will need more air flow. The limit switch is much higher than this number however and you should not be anywhere near hot enough to trip the limit switch. If you are measuring over 140 degrees or so at the outlet it is likely the limit switch is doing its job and you need more air flow