Anybody knows why when I installed a 5" monster AutoMeter tach with shift light into the "tach" mark at my distributor it just created me a lot of problems??? When the engine is in 1,000 RPM without rev the tach works OK, Going up on revs make the tach going crazy. As a result I ended adding a new set of sparks.

Maybe a bad ground to the tach. Try connecting the ground wire to the battery, for a test. See if that helps.
Do you have a Chevy distributor?