advan8 vs. advan13

Hello NONMEM users, Because advan13 was recomended for both stiff and nonstiff differential equations, I used it for stiff differential equations. It appeared that some results looked too sensitive to a parameters representing a "slow" processes. I did not observe it with nonmem6. When I used advan8, the objective function changed from 10228.853 (the final value; diagnostic plots looked good) to 5512.594 (the first value; I im still waiting for the final value of the objective function). Does it mean that advan13 should be used with caution when the equations are stiff or advan13 cannot replace advan8? Thanks, Pavel