I have a 1998 high efficiency NTGM125EKA that has just started to intermittently stop heating but continue to blow during its normal cycle. I've been able to get it back to a normal cycle but shutting it off at the thermostat for about 1/2 hour. When trying to see what's going on when I notice that it is blowing cold air, I turn it off and wait until it stops to blow, and then turn it on again and watch through the inspection window. I see it ignite properly, and then main jets turn on and hear the blower kick in and then watch the main jets go out and the pilot remain on.
Any help in how I could shoot or fix this would be greatly appreciated.

Sounds like the main limit is tripping. Did you check the return air filters,evap coil,blower wheel to make sure you had no air restrictions?

Yes, thanks T-Top, all are in good workign order. Since I powered down and recycled after about 1/2 hour, it has been working wothout failure the remainder of the day. I suspect a sensor somewhere between power and gas. Got any other ideas?

Regards