Wrong distributor for 69 318?

I have had a few point distributors over the years do what you are describing, I suppose an electronic one could do the same thing. The vac can rotates the point/pickup plate around the cam/reluctor on a pivot. When the pivot gets worn and the vac can pulls the plate the point dwell goes out the window and the engine runs like crap. The same could happen with electronic if the air gap gets way out of whack, but i have never run into that.

If you have points watch your dwell angle and apply the vacuum can, with electronic check your air gap with and with out vacuum on the can.... you will probably be able to see if something is wrong