I have an older 1990 tempstar furnace that been acting up as of lately. I know its older and I don't want to replace it. anyways what it does: Burners light, fan switch hits temp for fan to come on, fan comes on. Both run but the fan switch will hit the high limit and kick off the burners, fan still runs. fan switch will get to lower temp and kick the burners back on. cycles like this until it hits thermostat temp. it has new filter, no new ducts or returns added. What could I look for to be a possible problem.?

I'm new to the site, and came across this handy place while searching for an answer to the problem I'm about to detail. I just bought a house, and it has a gas furnace which checked out in the initial inspection. I work in HVAC, but specialize in geothermal units primarily, so I have little experience with natural gas heat. The unit maintains the temperature that the t-stat is set on, but I have noticed that it has started kicking the combustion blower and the ignitor on, almost like clockwork, every 10-20 seconds until the t-stat calls for heat, after which the unit kicks in as expected. Then when the temp is satisfied, it goes back to the combustion/ignitor cycling. I've seen several threads that discuss how to deal with combustion blowers that cycle in this manner, but none that also have the ignitor element heating up as well. I'm not currently living in the home yet, so I don't have the make/model on hand, if anyone could make use of that information (which I could get tomorrow), but I'm just trying to get a feel for whats going on. As of right now, I decided to shut the unit down until I know more about what is causing the problem. I appreciate any direction in advance.