losing coolant

I am having coolant loss from expansion tank overflow due to high pressure in the coolant system also the car runs at normal temperature up to 60 mph then starts to overheat if I take my foot off the accelerator it returns back to normal temperature I have replaced the cap ,thermostat, radiator and water pump and I have had the system pressure tested it holds pressure with no loss. also, a sniff test which no gasses were found fan also works fine. I am at a loss any help would be much appreciated.