transient simulation accuracy

Hi, 

I am using an ideal delay cell in analoglib. With defined 1 ps delay, the cell is sometimes behaving abnormally with changed slopes. The smaller time strobe and max time step may solve this problem. 

Is this error an systematic offset in the simulation or it would change with the simulation state ? And is this cell similar to the real CMOS transistor, which means that in the transient simulation, the  transistors may appear same artifacts in the simulation ?  And what would be a correct time strobe set for 1 ps delay ? 

Thanks.