Vermont castings double-sided vent-free. Had an older style SIT 820 (smaller diameter knobs) working great for over 10 years. System recently crapped out. Did some measurements and replaced the pilot assembly. No joy, so I went for a new valve. Thermocouple puts out ample mv, so much so, that it can hold this valve open and the old one (pulled it out of the valve) together as a test. Thermopile also putting out ample mv. Main valve clicks nicely when pilot is running and I toggle switch. So here is the problem... pilot lights and can run uninterrupted without an issue. All mv readings are good. All connections are good and tight. When I turn the control knob from pilot to on, the lockout lever inside drops into place as it should, but when I take my hand off the knob and it retracts a bit, the pilot goes out, but not on safety. Thermocouple temp drops, mv output drops and solenoid clearly lets go and spring closes inlet valve about 5-10 seconds later, clearing the lockout lever and allowing a restart. So, I confirmed pilot works, thermocouple is good, thermopile is good. I don't see any gas valving relation to the control knob. That seems only for pushing the inlet valve solenoid into place so the electromagnet can hold it. This makes me think that the next test should be gas inlet pressure (I have a manometer). Is that a possible cause? I have a gas boiler and 2nd fireplace on the main line and they work fine. Is this something that may be restricting the line in this branch so there is only enough gas to run the pilot? I guess I need some pointers from the pros. Right now I'm temporarily stumped. Thanks for any guidance.