Tachometer

My tach (85 2.5l swapped to 86 tbi and computer) has been fluctuating since I dropped in the new motor. I checked all the grounds and even changed the battery ground wire (was due anyway). It helped some but it still flutters up and down. Any more hints? What would I look for on the multimeter? Is the tach reading resistance, volts or frequency? I guess I should know but never had this problem before on any vehicle except my 260Z which had a gremlin in it. The imp, not the car.