must cycle furnace to get warm air

[facial]

Hi,
I have a an older Whirlpool Tightfist furnace with electronic ignition and I've
started having trouble. The unit will blow warm air for a day or an hour then it will start blowing cold air. We turn the unit off at the thermostate and leave it off for 30 seconds or so and then it will start blowing warm air again.

Any Ideas??????

Thanks

[lonestarheir]

Any further equipment information would help.