I have a Rheem Classic 90 Plus furnace. Lately it has been shutting down, normally at night and sometimes just switching the power off then back on is enough to get it going. At other times turning the power off and on will do nothing and then the furnace will start working again on its own several hours later. We had a technician in and he checked the furnace over and said because it was intermittent he couldn't do much, cost $60.00 When the problem next appeared it was a little different in that the burners would fire up for a couple of seconds then shut down, then fire up and shut down, then the flame light would flash on the control board. I cleaned the Remote Sensor and the spark ignition and sensor and it seemed to work for about 4 days before it shut down again. Now it just tries to cycle on (no Burn) and then shuts down, keeps trying to cycle on and at some point is successful as the furnace eventually starts and runs. The other evening it shut down and it was about 40 degrees when I got up, trying to recycle the power did nothing. We called a technician right away, but he showed up 5 hours later and by then the furnace had restarted. He checked the inlet and outlet pressures which were OK and he said the air inlet pipe was slanted downward going outside. He corrected this so now it is slanted up going outside. That cost me $110.00 and the following morning the furnace had once again shut down. Recycling the power switch got it going again. The inlet pipe is 4" PVC which has one 90 degree elbow and then reduces to 2 1/2" PVC going outside, total length is about 6 feet. The outside inlet pipe is horizontal with no 45 or 90 degree elbow. There is a moisture hose on the 4" PVC inlet and there is water in the trap. Could wind or snow blowing into this pipe cause the above problems? Any other suggestions, it is cold up here and local technical help is limited where we live?