[H'S944]

bought this car after the prior owner had the timing belt break. just rebuilt cylinder head and replaced gasket water pump and thermostat bled air out of system. now have two problems. Temp sits at 3/4 mark after warm up and stays there until driving and stuck at lights goes up to red marker and makes its way back to 3/4 mark. now also drops rpm to 400/500 occasionally now and again stops completely but will re start. highway speed runs like a dream temp just below 3/4
any ideas please. I was told it didn't have these problems until the belt broke and it bent ten valves.