Suburban NT 16 seq furnace. Yellow flame. help!

I have a brand new suburban NT 16 furnace that I installed 2 weeks.ago in an enclosed trailer I converted into a camping trailer.
At first it seemed to work fine,  but last weekend on a trip I fired it up upon reaching our destination and it was fine at first but after about 2-3 minutes of running and being outside I heard slight what I can only call a ?rough air noise? at the exterior exhaust port. I also noticed the orange/ yellow flame at the burner window. The furnace then really struggled to warm up the trailer. It was single digits outside, but the furnace had never had a problem in similar temps while testing it.  I shut the furnace down for the night and in the morning found a place and purchased a new 2 stage regulator. It seemed to help the first few times the furnace ran for the next couple nights but the temps were much warmer 20-35 degrees. Yesterday I heard the same air noise and obviously the regulator wasn?t the issue.
Tonight at home with temps again near single digits and the trailer cold,  I fired it up and sure enough after about 5 minutes of running it was back again. I was at the exhaust tonight and I put my hand over the fresh air intake and held it there and within a few seconds the noise stopped I went inside and the blue flame was back. It was fine for about a minute and then as I was outside again I heard the ?poof? and the noise immediately came back. No easing into, or slowly getting louder  it was just there and of course inside there was the orange yellow flame.  I can repeat this scenario over and over.
Any ideas what could be causing this before I take it to my local dealer to test??
Im busy at work and need to use the trailer (and heat)  this weekend so if the dealer can?t look at it I might have to find time to look at any possible solutions myself.
Any ideas?

Tough one. It may be that you have a bit of moisture or glob of oil in the propane lines (after the regulator) and it gums up when its really cold.  Maybe disconnect the furnace LP line and let the propane blow though fr a few minutes to try to purge it?  Don't light a match!  Or disconnect both ends and blow compressed air through?  Of course, whatever it is may have worked its way to the gas jet in the furnace by now, but the furnace probably wouldn't work at all in that case.

Is there a removable port at the furnace gas entry where you could measure gas pressure there? Or maybe could insert a Tee to do that?

I was able to look closer at it tonight. I pulled the burner tube and it was very clean.
I disconnected the trailers gas lines from the furnace and hooked it up to a separate propane tank with a line and regulator. As usual it ran about 5 minutes and the popping started. I was able to figure out that blocking the exterior fresh air vent simply killed the flame and when I took my hand away the igniter fires the gas back up running as normal with a heavy blue flame.
I was able to sit and watched the burner tube window to see what was happening I even took video though not sure how to upload it here.
What happens is the flame almost completely extinguishes itself . Fflashes orange and then goes into a broad blue flame with orange top. the heavy blue flame at the burner tube is gone when that happens. A few times it ?flashed? but kept operating normal but within a minute or so it flashed and the light blue orange flame and the rumbling noise began. The flash happens and there is almost a fire ball inside the burner and zero flame at the burner tube grates.
My local dealer can?t get to it until next week at the earliest and frankly I?m at a loss.