Tach Messed Up Reads 0

I hope this is in the correct area. I had to have the NSBU replaced bout a month ago, so doubt thats it. Today I was hauling my 6.5 to get the head gasket replaced, in the new motor that was just put in, in July. Anyway my tach on the '03 DMAX started acting up. Starting out it would read up to about 1500, and with each shift it would drop down till it read 0 in OD. First tried shutting off and cycling key couple times, seems that a few other proplems are solved this way, to no evale. I did notice in the couple days leading up to this failure that the tach was "JUMPY" it wouldn't move smoothly. Just say that it would read 1500 then jump to 1700, not move up as it should. I did a search on this and saw something related to the alt, could that possible be it, I haven't had any alt alarms? Even though its an '03 I only have 33??? miles on it. Starting to get fed up with the whole vechial thing, think I my go buy a horse. Thanks in advance for your input.