Car problems?

Hi,
I need your help if possible; I have a Rover 45, Y reg, 1600cc K series Engine.
The engine had a new head gasket fitted with new stretch bolts and all was well for 6 Months, now the engine keeps overheating and the fan kick in toll reduce the temperature.
The matrix sometime change from hot to cold, warm then back to hot and now the engine is losing power in gears 2,3 and 4 the returns to normal.
There is a high screeching noise that as soon as it sounds it disappears before the power loss.
I am toping up the water tank about ¼ pint a week and the oil is still clear and clean without any white slush.
Has anybody got some words of wisdom to explain what is happening and how I can fix it.
I am not a mechanic so please view this in mind, many thanks for any help!