1979 Chevy Malibu 1.5L of oli disappear every month

Engine Mechanical problem
1979 Chevy Malibu 6 cyl Two Wheel Drive Automatic

Hi
I have a 1979 Chevrolet Mailbu that has a new engine
a 4.3L V6 the same engine that come in a new GMC safari but in the version with a carburetor.
it has on it no more then 100 thousand of kilometer
which is nothing for that type of engine.
now a few month ago I have ask here a question as if I can move to unleaded fuel as the government has stop selling the fuel that I was driving on that was
96 octane that has lead in it.
i was told that I can move to the new 95 octane that is unleaded. So I did move and all was ok until now
this is the second month that in my routine check I have found out that oil is missing from the engine.
so I have add a stops leak liquid and more oil and now a month later again 1.2L is missing now there is no oil leak that I can see on the floor.
so the only explanation is that the engine is eating it
now yesterday a new problem has appear if I am with AC and stand with brake and the car is in D suddenly the engine collapse and die now this happen when the engine was cold and also when it was hot so temperature has no impact on that.
now to add more info on that sometimes then I close the switch to close the engine it don't what to get close and I get a few hiccup from the engine before is get close.
this start after I have change to the new fuel.
but besides of all that the car run smooth as it was.
so what can be the problem with the oil and all the rest, I will go to a workshop but I want to get your info so that I will know if what they will tell me is true of if they only try to take my money and ruin my car

p.S
I am from Israel.

Yours

Tamir