[racing green rover]

Hi sometimes when I start car up and let it idle the top hose and the bottom hose get hot as they should then when the fan kicks in it will be on for about 5 minute and all of a sudden the bottom hose will be cold if I rev it the hose gets hot again and the fan quickly turns off at 95 degrees this happens sometimes not everytime i've recently replaced the oil cooler and the lower coolant rail as the cooler was blocked with k seal when I bought the car 12 month ago i have since flushed all the cooling system countless times and replaced the thermostat about just under a year ago. The running temperature is around 90-94 degrees on an open road however driving slowly around built up areas and stopping at traffic lights it can rise to between 98-100 the heater gets red hot. Forgot to say I took it for a run earlier and when I got home I checked the bottom hose and that was hot the fan kicked in and went off in about a couple of minute.
Any advice thanks it's a 2.5 V6