GG has some very good points about having another tech come out. The gas pressure should be checked at the inlet and the discharge sides of the gas valve. In addition to that, you possible could be cycling on the limit, meaning that your flames are on again and off again. They should be steady until the call for heat has been satisfied.
Let's be gentle with the tech that was out there....we all were new or learing at some point. The problem with this one is that the tech should have called for help from the shop.