timing problem

[-the.k.w.c-]

i have a s14 sr20det.

problem is every time i set my timing to the 2nd notch from the right at 15 degrees about a week later sometimes less the timing will be off again some times to the right of the notch some times to the left and some times its not even on a notch ...what could be the problem? also some days my car will back fire ..some days not

[240sxspeed]

is yuor idle different when u check the timing and what methofd r u using to check the timing