Goodman not lighting up in the morning especially when really cold...

I have a Goodman GMH950904CX furnace that occasionally does not light, especially on cold mornings. Single blinking LED error code. Have replaced pressure switch several times. I even rerouted the vent pipe since it was installed without sloping straight sections, and even runs down hill through the wall collecting water. But had same issue again this cold morning. Not sure where to go from here. Purges fine until glow plug is bright. Then gas valve does not open. Tapping pressure switch helps many times. But I tried changing the pressure switch, and the valve opened and closed repeatedly, like pulsing every second. Put old pressure switch back in and it lit fine. I did notice a slight pulse just once or twice before staying lit. Any ideas will be appreciated. Thx.