Is the furnace natural gas or propane? I am having a hard time with an adjustment of pressure from 15" to 6". I am assuming you're talking about the regulator at the gas valve/meter to the house or at the propane tank, not the regulator at the furnace gas valve. Did the tech measure the pressure on the manifold side of the gas valve? It should be 3.5" w.c according to the installation -
w w w .xpedio.carrier.c o m/idc/groups/public/documents/techlit/58pa-10si.pdf (remove spaces as necessary)
A gas valve is a simple valve except it is opened and closed by a solenoid. What can happen is something interrupts the current to the gas valve and it will close. I would try and watch the furnace in hopes of seeing the flames extinguish and relight. At that time you want to carefully listen for any metallic clicking noise indicating the gas valve did, in fact, close briefly.
Since you say the flames extinguish then relight, I would check your filters and make sure every register and return are open and clear. If the furnace overheats it will shut off the flame until the heat exchanger temp is back to normal and then relight the burners.
I would make sure the power to your thermostat is solid. If you have batteries only, replace them. A weak thermostat can cause interruption to the heating cycle.