Hot Water Shot?

We bought our house in 2009. The hot water heater has always given us trouble. Mainly the 40 gal gas tank installed in 1998, will not keep hot water hot without constant use. It may be due to the fact that we have to keep the temperature so low or it will boil over leaving the floor soaked and me worrying the tank might shout out of the house. We normally have to keep it in between vacation mode and suggested hot mode. Is this tank shot? I had a professional come out and he said it was fine and just use it till it dies but the water sometimes comes out so cold when my one year old daughter cries during bath time.