And yes most units that I am aware of come with a fast acting valve. I also know of furnaces and boilers where replacing a slow opening valve with a fast opening valve scared the heck out of me when they lit off! Try a fast opening valve on those old Bryant boilers once or for real kicks on those furnaces that used those goofy trumpet like burners.
I may be wrong. Maybe a fast opening valve will solve the problem. Why the furnace suddenly needs a fast open valve surely should mystify us I would think. The op said he replaced the gas valve. Unless there is a known issue with this models ignition I hardly think that out of the blue these units woke up and decided that slow opening valves were a poor choice.
While I am fully aware that many of you folks have as much or more time in the field than me I just feel that suggesting one to alter a units design is not the way to go.