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the reason the timing is jumping is because you have to disable the computer controlled timing when you set base timing. you have to remove the spout (spark output) connector. this is located on the drivers side of the engine compartment close to the hood spring area. it is a small fingertip sized connector you just pull out then set the timing to 10 deg btdc.
hope this helps
i have done that too, but now it wont start.
another expert sent this to me to help and maybe this is your problem.
If the truck seems to be running fine,and the only problem is the timing mark seems to wander,it is very possible the timing light clamp is picking up stray voltage from other wires and confusing the reading.or the light is malfunctioning