Transmission not holding on hills

He meant when the truck lurches, is it forcing the engine to turn over or slipping the clutch. Just a guess here, but like someone said above if a valve to a cylinder is open there is no compression in that cylinder to hold it. So if say 5 cylinders have a valve open thats only leaving one cylinder to hold the truck. Then when the truck starts to roll, it turns the engine over and you get compression in other cylinders that in turn stops the truck. If the hill is steep enough it can cause enough momentum to roll through the compression and become free, hence the lurch, stop, lurch, stop.
Again I am just taking a stab at it. I am no mechanic, nor do I even own a manual currently. This is just what it seems like to me.

Edit: SC4333 beat me to it. I type too slow.

 

Not a problem - I'm learning something new and that's good for old dogs as well. As I said, I guess I've been lucky to date. I was taught it was sufficient to leave a truck in gear while the engine is off so it won't go (or roll) anywhere. This obviously doesn't apply to steep hills but that wasn't what I was talking about here.