I have a very old rheem (15 + years) gas furnance/AC in the attic. Lately, my home has been very cold, the first time in the 10 years I've been here. I don't know much about hvac but here is what my unit is doing. First, it is cycling on and off nearly constantly. It will go thru one heat cycle and after maybe 5 minutes start again. When I turn the thermostat on, the burner comes on and heats for about 3-4 minutes and then turns off. The blower comes on after this. The air is warm but not like it used to be and the blower continues until the air is cold. Then the blower goes off, the burner comes back on and the cycle repeats constantly. The unit has no electronic parts, it uses a pilot light and a fan limit switch. The limit switch is set to off at 90, on at 130. Is the limt switch bad, do I need a new thermostat?
Any information would be helpful. Unknown model number

Thanks

Limit switch would be were I started my inspection, that and the filter.

Remove the cover from the fan/limit switch and observe it with the heat running. Does the dial rotate past the high temperature trigger? If so, the unit is overheating. Try setting the thermostat fan selector switch to the "on" position and observe what, if any, change occurs.