Could he be getting into food somewhere? The trash, maybe? He actions sound like a dog with an upset tummy, he needs to throw up and eating grass is what makes them do this. Dogs are quite intuitive about what they need. Are you using a treat once in a great while that is not agreeing with him? That could be it. Since the episodes are so spaced apart, it is not a food he is getting daily. Maybe a relative or friend visits now and then and brings over something for the dog -- or even something for you and your family, and you give the dog some scraps?

I really think this is some food he is eating which is not agreeing with him.

Another possibility (just thought of it!): is this happening when he has been left a bit longer than usual? What is he throwing up, is it food or just yellow bile? Could be he is going too long with no food, and stomach acid builds up giving indigestion. Again, the dog will eat grass to throw up the excess bile. Does he readily take a treat or a bit of food after he throws up? That would also mean it is an empty tummy. Older dogs, like older humans, need to eat smaller meals more often, as compared to what they used to do when they were younger. Maybe this is happening when you are going out after work, so his meal is delayed a few hours?

Those are a few things to think about. Hope it helps!