What do you mean your car doesn't have a tach?
Your vehicle should have an idle speed adjusting knob to the left of where the steering coloumn passes through the dash. I don't think this is for use in warm weather, really, but I suppose you could try cranking it up some and see if your engine still cuts out.
Have a reputable MB shop check the compression on your engine.
It's possible your engine is just getting really bad compression and this is the reason it cuts out at idle.
