Tachometer is a little jumpy

Ok, i just got finished with my motor swap, and i decided to put in an autometer monster tach with shift light. When my car goes into vtec, the new tach and the stock tach jump all over the place (the new one doesnt do it as much, but it still happends). The shift light seems to still be accurate, but i think the problem has something to do when the vtec kicks in. What could be wrong?

Thanks a lot.