Gas and Oil Home Heating Furnaces - Gas pach shuts down before thermostat setting is reached

I have an approximately 5 year old Trane Gas pack. The problem I have been having is this: During cold weather (usually in the 20's) the thermostat will be set at 70 however the temperature will only reach 68 or less in the house and the unit will shut down. Sometimes in the morning the thermostat will still be set at 70 however the temperature in the house could be 62-64. I can reset it and will work for awhile but then the same problem will reoccur. The service guys have come out and changed filters thinking it may have been an air flow problem. Then then ran more duct work to distribute the heat better. Finally, they changed the thermostat. To date nothing has worked. I've been reading the community forums and see where folks have had similar problems. I would like to be able to offer some suggestions to my service guys in a way that would not have them offering some suggestions to me. Any ideas are appreciated.
Thanks
Chuck

I have an approximately 5 year old Trane Gas pack. The problem I have been having is this: During cold weather (usually in the 20's) the thermostat will be set at 70 however the temperature will only reach 68 or less in the house and the unit will shut down. Sometimes in the morning the thermostat will still be set at 70 however the temperature in the house could be 62-64. I can reset it and will work for awhile but then the same problem will reoccur. The service guys have come out and changed filters thinking it may have been an air flow problem. Then then ran more duct work to distribute the heat better. Finally, they changed the thermostat. To date nothing has worked. I've been reading the community forums and see where folks have had similar problems. I would like to be able to offer some suggestions to my service guys in a way that would not have them offering some suggestions to me. Any ideas are appreciated.
Thanks
Chuck

Horrors to people who hire repair people who can't diagnose and just wind up changing out every part they can think of.

You have to tell us a little more information on what the behavior of the furnace is when you get it to call for heat. Does the ventor always come on? Does it always ignite? Does it always fire up and stay lit for more than 2-4 seconds? Then we can go from there. Take off the main big cover and look and see what you can see.

Thanks DaVeBoy. I'm not sure about the ventor always coming on but I can tell you this: it always ignites and stays on for more than two to four seconds. It stays on for awhile but in cold weather never seems to reach the assigned thermostat setting. Like I said this does happen all the time only in cold weather( in the 20's) For instance the temperature this morning in the house is right where it should be. However tonight we can have it set for 70 and it will only get up to 67 or 68. Not a big difference but enough to have my wife and kids wrapped up in blankets most of the winter.

You have to find out if the thermostat is shutting off or not. You can't just go by what a thermostat reads. Especially when it is within just a couple/few degress of the set temp. Thermostats can be out of calibration that much.