I would bet that you are not powering the valve with a PWM signal and are instead getting an increase/decrease signal.  The PV70-30* cartridges are pretty bullet proof and commonly fail as not opening when something lodges in the spool.  I agree with tedbear that if you are driving it with a PWM and it is stopping occasionally, then I would want to see what your PWM output is.  What you are describing with the pause seems consistent with over driving and then pausing to allow the driver chip to cool.  You should try reducing the current to the coil by adding an inline resistor.  Dropping voltage across the coil will reduce current and cause the valve to go to a partially open state; the PWM driver is modulating current by controlling percent of time ON vs OFF.