A new method has been developed to calculate the perturbations caused by passing stars on inner core comets. It is a modification of the impulse approximation in which the star path is split up and several partial impulses are calculated, rather than taking a single impulse at closest approach. The new method is about 10-100 times faster than numerical integration, with which it gives good agreement. In a second line of investigation, it is found that, for small changes, stellar perturbations are as likely to reduce cometary perihelia as to increase them. Because the magnitude of a negative change in q is limited by an absolute boundary at q=0, a small number of very large positive deltaq values wil create the appearance of a positive tendency in changing q for many successive stellar perturbations.