1997 Mitsubishi Galant Car stalling every 15 miles

This car was taken in the shop for overheating issues on the way to a 2 hour trip. The mechanic diagnosed it as a leaking freeze plug on the rear of the head. The plug and a radiator cap was replaced. On the way back home the car started stalling at about 15 miles of drive time. The car would start right back up but would stall 15 miles later without fail. At one point the needle on the temp gauge was 3/4 to the "H" mark. We will be taking it to a mechanic locally but would really like some idea what could be the problem.