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The 4.9 in my truck is skipping/jumping timing. I can set the timing and get it idling good, but when I try to drive it the timing jumps way off. Timing gears are good, dots lined up, distributor hold down is tight. This same engine/ignition combo has worked for a couple years, so it isn't new parts not matching. Any ideas? I haven't checked the distributor drive gear roll pin yet, could this be cam "deflection" from worn bearings (132k on motor)? The outside of the dist. isn't moving, this is all internal.