Man, I feel that in my soul. Intermittent stuff is the worst because everything looks fine on the meter until the plane decides to flex and mess with you. Glad you finally found it though, one pushed pin can make you question your whole career for a few hours.

Did you check the flue size before buying that flat one, because on a 6 inch liner you might have been fighting the wrong shape the whole time? Round makes sense on square tile but I never see guys talk about matching brush shape to the actual flue, they just grab whatever is on the shelf.

Wait, are we really calling this a win? A blown donut gasket usually means something else is going on, like a warped flange or loose bolts that let it cook in the first place. If you didn't check the mating surfaces or torque everything back down right, that $4 part is gonna blow out again in a month and you're back under the truck. And honestly, "almost handed a shop $300" but you still spent 3 weeks listening to it tick, so the savings ain't quite as clean as it sounds. What made you so sure the manifold was cracked before you even looked?