Payne furnace 6 years old ran flawlessly until a week ago. Locks out due to no flame present. Random ignition as follows. When valve opens and first burner only lights, next four will light in time for flame sensor to register flame present, even though there is a noticeable delay in ignition. This happens maybe one out of 6 or 7 attempts. When first two burners ignite (almost simultaneously) last three burners will not ignite and furnace shuts down due to no flame present. This is most common scenario.
Outlet gas pressure starts at 2.5" and slowly raises to 3.5 after approximately 4 seconds ( have to manually light burners to take this reading). Inlet gas pressure was 6" static and dropped to 5" after valve opens and lights. Replaced gas valve and problem still exists. Gas company changed meter and regulator and now inlet is 7" and drops very little when manually lighting furnace, but problem still present. Going a little nuts now, so I recheck everything including manifold alignment and all looks good. I just can't get the slow opening OEM valve to open fast enough to light all the burners before lockout. Any ideas?