Jotul 3 Classic NG Problem

I have a Jotul 3 Classic gas (NG) B vent stove. It's about 10 years old. During the last season it started burning very rich and sooty with crud building up below in the burn chamber. I'd clean it out, run it another month, and end up with more soot and crud. When I say crud, it was piled up right to the burn tube sometimes. So I first opened the air intake near the base of the burn tube to see if this would balance it. I then realized the burn tube itself was completely corroded and cracked, and with a lot of trouble from Jotul replaced it (it is the one piece "U" type tube and it cost almost $200). All this history brings me to the problem and the question....

Now the stove lights but doesn't stay on. Furthermore, without any change to the gas pressure (which is normal) the stove is totally over fired when on with huge flames. Nothing has been touched on the stove valves and the new tube is OEM perfect. I have trimmed the gas pressure with the house gas line's valve so the flames are normal height, but the stove still kicks off after a few minutes. What is going on and did something happen when it was sooting heavily to burn out the stove's gas valve?

I should mention that I have cleaned thermopile, thermocouple, and bypassed the spill switch (just to check it) and they all seem fine.

Thanks for any help.

Is the vent cap clear of obstructions? Very strange that its overfiring, the manifold pressure might need to be checked. Check to make sure the orfice is still seated on the feed tube, or the feed tube is not cracked. Do all the things Dak asked and report back?

I know this thread was posted a long time ago but I wonder if you ever found out what was causing the problem. I bought an old Jotul, Classic DVNG D31719 that had never been used. When I turn the flame up, after a couple of minutes theres a click and the pilot light goes out.
Has anybody got a manual or installation guide? Wonder if it was installed right.

is the unit spilling exhaust?? smoke(insense stick etc) near hood should get sucked up chimney...
if so the spill switch/ and or venting sounds like the culprit
bypass it for testing only!

i have generic troubleshooting info avail, does your unit use robertshaw or nova sit valve?