Tach problems?

[cclark746]

My truck idles down to around 500-475rpms sometimes but it doesnt seem that it is idled down that far? i can idle it up either by my high idle feature are hold the brakes and and slowly press the gas pedal and it seems when the tack hits 850-900rpms it jumps up about 200rpms. Has anyone had any problems with the tack readings, are is there something else I should check out are be concerned about. I am currently running the smarty revo on timing 4, rail 3, tq 2, catcher 3 thanks any help are advice will be helpful.