With sloppy springs in an old distributor I had a couple cars die after freeway runs. The advance mechanism makes the rotor come too far back after high rpm's and retartds the timing and kills the car. This, even on distributors that have good springy action in the correct direction. Another used distributor cured the problem. I usually run my B20's at 10 degrees so maybe that's not the same thing you're seeing. What does your gauge say (like we trust thirty year old gauges), anything different than normal?