As a newbie, one more item I am having trouble with on my 96 PS is the tach. The tachometer will fluctuate about 600 - 700 rpm when I am cruising. I can be travelling along at 70 mpg and will see the tach jump from about 2,200 rpm to about 2,700 rpm (I have 4.10 gear). Anyway, I am 100% sure it's not the tranny going in and out of lock-up or shifting in and out of gear.
Any ideas what may be making it "jump". Some days it will read on the low end all day, other days the high end, and some days it is irratic. I assume the tach is electric but wonder what signal drives it??