overheating when driving at 60

so i do deliveries in my rover, the cars fine on the 20 mile journey to work and throughout the 5 hours of work during the evening. On the way home after about 10 miles the lights flicker and then it starts to overheat comes back down to just over halfway when i go back down to 50. sometimes you can hear the coolant pressurising and it will also be right at the top of the bottle. iv recently replaced the img and thermostat although didnt use genuine parts. i have used steel seal in the engine also. Any advice on my probably cracked head appreciated

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