I have a 4-year old T-K2 that is starting to have the same issue as the first post. My assumption is dirt since I haven't cleaned the unit since it was installed. (What can I say? If it ain't broke don't fix it. Plus I have a whole house water filter.) I'll post what I find out.
It took me awhile to fix. I started with cleaning the fire box. Then the symptoms were as if the flame sensor was not working. It would spark and light off but after a few seconds it would it would shut-down amd recycle. So I replaced the flame sensor and ignightor since I noticed the gasket was torn and missing 1/2 inch of material. Same problem but when it did light and I gave the unit a high demand it would blow itself out. So the tech on the phone told me it was probably a gas supply problem. They were right. I replaced the gas supply line since it was undersized and got new high flow ball valves and replaced the pressure regulator. It's working fine now.