[DanEC]

Might still be too much advance coming in at cruise. Do you have a pretty quick timing curve or a lot of total timing in the motor like 40 deg? You might experiment by pulling a couple degrees of initial timing out of it or putting in the next heavier centrifugal advance springs to slow down the curve a little. The way you describe it as - a slight miss every 5 seconds or so - sure sounds like what I experienced in my Corvette.