Gas and Oil Home Heating Furnaces - Lennox SX 90 cycling

Greetings... I have a Lennox Ultra SX 90 (model GUK050D10) that has been working fine for years. The first time I tried it this fall, I turned the thermostat switch to 'heat' and upped the temp to 14 or so (ambient temp was around 8C). I heard the furnace fan turn on, and about 30 secs later I heard the igniter followed by the propane ignition. In other words, all seemed normal. The furnace heated my shop to the desired temp, and then turned off. So far, so good. But then I noticed that the furnace fan was cycling: turning on for about 5 seconds, then spinning down, then repeating the cycle ad infinitum. I must switch the thermostat switch to 'off' to stop it. If I try again after the shop has cooled down (or set the target temp up another few degrees), the same sequence repeats: furnace fires, heats, reaches target temp, turns off, and begins its cycling behaviour. The thermostat is a simple, non-programmable 3-wire digital Noma which has worked fine for several years.

Any ideas as to what I can check or do to fix this? Thanks!