What causes this??

[Ric83]

My dad asked me to chech the timing on his truck yest. ('83 Scotsdale w/ stock 305)
When I took a look at it, it sounded like it was idleing a little too fast. When I pulled the vac. hose off of the advance to check the timing, it made very little diff. as to how it idled. Well, while shining the light, I noticed that every few seconds, the timing mark would jump quite a bit to the advance side. What would cause it to do that?? I know his truck needs a tune up pretty bad so we will do that, but is there something in specific I need to look into?? After setting the timing at 10, I took it for a drive and it drives like a dog. It has not kind of power, you have to let it build up speed and forget about passing anyone. I ended up setting the timing at 8 instead. It seemed to like it a little better but had all the mentioned problems still. Anyone have any suggestions?