Is there a way for me to tell if I have a cracked engine block or is it a head gasket job I need on my 1991 300 SE

Symptoms: When the car is at a red light the temp guage goes up. When the light changes to green and I accelerate, the temp gauge goes back down. I was told about 3 years ago by a Mercedes Dealer that I needed a head gasket. This is a "second car" to me, I don't drive it that much. The problem has stayed the same, it does not appear to have gotten any worse or better over all this time. I am a bit hesitant to believe the dealer because I also have a Mazda and oddly enough around the same time a Mazda Dealer told me that my Mazda needed a head gasket and I have found out that was not true.

My 3000 SE is really starting to go through coolant. I normally only drive it once a week and that is just around the block but I find that I need to put coolant in the overflow every week. When I moved the car today I could tell that the coolant was leaking from further back and not by the front of the car. When I took the car in to the Mercedes dealer 3 years ago they also told me I needed a radiator and a water pump.

My concern about a cracked engine block is this. About 3 months ago I had taken the car around the block and I did not know it, but the overflow was bone dry and the car heated up and went into the hot. I shut it down right away. Since then, it has continued to do what it has always done. It runs ok until I am at a stop light then heats up but once I accelerate the temp gauge goes back down.

If I cracked my engine block - would the car even run?