I'd check your fuel supply. Particularly any regulators, look for kinked copper pipes, etc. Fuel supply problems often look like stepper problem.
You should have 10-12" of water column pressure not running, with no more than a 1" drop when running.
I only use an analog manometer, it tells you so much more than pressure.
Hook up the manometer and when you energize the fuel valve with a jumper, it should instantly move to 11" with no bouncing. Bouncing indicates, a long pipe run, water in the gas pipe, or regulator vent pipe, undersized piping, or a bad regulator.
The regulator vent may be restricted. Some people attach a pipe to the regulator vent, but if the regulator is outdoors this is not necessary. Remove any pipe from the regulator vent if it is outdoors.
The regulator may have water sitting on the atmosphere side of the diaphragm, check for proper orientation of the regulator vent.
Is the fuel tank level very low?
Etc. Fuel supply issues.
State your problem, not your diagnosis.