Got this truck dialed in nice, except it rumbles and shakes the truck about every 15 seconds of idle. When you monitor with the flashscan you can see that the timing is flaring up to around 30 degrees down to about 10 during the rumble then it clears up for another 15 seconds. Any ideas on this, is this the idle spark flare up that is causing it or is it just something that we have to live with.

Aaron.