sinking stationary gas

Hello people I am an absolute newcomer in things motor have but some questions to you point one has settled and that would be cylinder head seal compression is in order of the cars ran only on 3 pötten so I have yet another problem and that is when I start the engine runs the speed normal at approx.780-820 turns after a certain time this sinks but and the car starts to jerk in addition to it comes in the lower speeds it can not regulate well the heat sst until 1200 turns the speed decreases again and again starting from 1200 turn it runs completely normal and I can regulate it.What could it be that I have such a few ideas? then I have a problem with the temperature of the cool water I reach on the highway only a maximum of 65 -70 degrees according to the display in the city he comes to 82-85 degrees?? what do you say is the normal I drive the car first 2 weeks? and my cooler runs after each driving a long time after no matter if I drive 5 meters or 100km would be happy about any answer!!! lg ingmar