My water started going cold in the shower after about 5 minutes a month ago. We changed the elements in the water heater and it is still doing the same think. Could it be the dip tube my heater is electric and about 7yrs old.

If you test your elements and they are functioning then a failed dip tube is a likely suspect. Be careful when you remove the dip tube, they are very snug and you can twist the water heater by the torque of your wrench. I'd get a helper who can hug the tank to counteract your torque