Will a 95% furnace preform better than my 25+ year old furnace?

[wowzer]

How frequent is 'too frequent'...as in cycling on/off too often. Any rules of thumb? That is, in the dead of winter, the furnace should run for XXX minutes in a heated house (i.e. not a house with the termo turned way down).

What about AC in the summer? If the house is kept at 78, how long should the AC run? 10 minutes? 15 minutes? 5 minutes? Just curious...