Lennox el280uh090e48b-01 gas furnace

pittsburghgary

I have a new (1 year old) Lennox el280uh090e48b-01 gas furnace.  When it gets the signal from the thermostat to kick on , the furnace heat bar heats up then you hear the gas valve click on and then the furnace starts to Sputter several times till it finally ignites. Has done this for a couple months.  I had the HVAC company that installed it come out and check it out. They said they've never seen this happen before.  They did a few tests and couldn't find any thing wrong.  They ordered a new gas valve and circuit board under warranty.  They replaced them. Still did the same thing.  We watched and listened for a week,  they came back out. They couldn't get it the Sputter once for them. They voltage tested every wire on the harness. They even thought that the humidifier could be causing a voltage drop, but it wasn't at all. 
One of the techs said that the flu pipe wasn't getting very hot and that the main flu to the roof (2 story house) might be oversized ( it's original (8") ). Furnace and waterheater are 4" to the 8" main flu pipe.
As a homeowner I am looking for some advice on what anyone thinks (no wiseasses) , the tech said that there might not be enough back pressure if the flu is oversized. He thinks it should be @ 6-7" pipe. The current 8" pipe is single wall galvanized pipe, the 4" from furnace and water heater are double wall. 
Thank you in advance for your assistance. 

JUGHNE

I would monitor the gas pressure to and from the gas valve.

Possibly the gas delivery regulator is doing the shuddering.

It may be happing in the water heater and not noticed.

This theory applies to NG or LPG.