The pump might be failing or the controls may be malfunctioning. There is more than one way to set up the circulator/boiler/thermostat and it will be difficult to tell what might be wrong without knowing how it is supposed to run. What is the boilier doing when the circulator is stuttering?

The line has a check valve somewhere that probably is stuck open. I would look right where the line connects on the pump or right below that fitting as some are replaceable. If the check valve is stuck open as soon as the pump kicks off the water that was pumped out runs right back in and cycles the pump.