First of all, I've seen Why does my moka pot sputter?, which covers a very similar issue but has no answers and doesn't ask the specific question I have.
My 6 cup Bialetti is probably from the 1960s, but the modern gasket still fits and I've replaced it before (over 10 years ago, but I don't use it daily). It was blowing steam from the screw joint so I checked and replaced the cracked gasket. With the new one, I get about a teaspoonful of thick coffee before the sputtering (that normally marks the end of the brew) starts from the tower. If I leave it sputtering I eventually get a little less than I should, but enough to drink. I don't normally tamp it, but smooth over the grounds with the back of a teaspoon before wiping the sealing face of the basket with my hand. I use a very small gas ring, with flames smaller than the base (a little smaller to start, reducing once the flow starts).
Assuming there were grounds caught in the seal, once I took the seal out and cleaned it, before assembling wet. It behaved much better, but only that time. I normally only rinse the pot (it's very well seasoned) but I'll give it a good scrub before I next use it. I wonder if the seal doesn't seal very well dry, and whether wiping a thin layer of neutral cooking oil over it would help. Is this a good idea?