I am sorry to hear that you are still having problems. I try to help people out by giving them the easiest and least expensive solutions to their problems that I have seen over the years in my HVAC business. Sometimes other components, parts can cause problems. Sounds like you have an occasional problem that is going to be hard to find because it only does it sometime. You almost have to be there when the problem occurs with a meter to test and find out what the problem is. Many times the furnace control board will give you a flash code telling what the problem might be. I would suggest if your furnace is equipped with a control board that has a flash code that you read the code the next time the problem occurs. This problem could be a safety control problem (pressure switch, limit switch) a loose or bad wire connection, or a control board problem. Best of luck in finding the problem.