TRANE XV90 PROPANE FURNACE PROBLEM:

Approximately twice a week my trane XV90 high efficiency propane furnace (10 years old) will blow cold air, the thermostat is calling for heat and the fans come on but the burners do not light. If I do not intervene the fan will continue to blow cold air for about 30-45 minutes and then the fan goes off and after a few minutes the furnace will recycle and the fans come on, the burner lights and everything is fine. If I intervene while the furnace is blowing cold air I can turn the power off wait a few minutes and turn the power back on and the fans come on and the burners light and everything is fine again.
The furnace has not yet failed to start up on it's second try but I am concerned this might happen. I had the local service professional out to check the problem and he cleaned the ceramic ignitor and also cleaned the burners with a wire brush. This did not change the problem so I installed a new trane ceramic ignitor and needless to say this did not help either. I had the same service man out again and he ordered and installed all new burners and this also did nothing to change the problem. Now the service man has indicated he would be unable to diagnose the problem unless it was occuring while he was here.

I need HELP-- Has anyone out there had experience with a similar problem or have a clue as to what might be causing my problem. Any help would be greatly appreciated.

snak1e