I've been having the same old tach problems with my 06 until I took the crank position sensor off and cleaned it then the dang thing went all the way around and stayed there
A few days later it was still stuck on the back of the needle I was pretty much about to give in and buy a new cluster until I installed the new towing brake controller I just got. I finished the install and fired the truck back up to hook up to the trailer and what do you know the tach is sting a idle and its been working for the last week which is the longest time its worked properly since I bought the truck.
Still trying to figure out what wire I hit or what the heck happened, Its just weird...

 

dont you just hate them kinda problems

 

My brake controler is plugged into the factory supplied pigtail but i think i will un plug it when i get home and see what happens. might be on to something here because my cluster has been replaced once already for this.

 

hmmm thats interesting

 

im giving it a try also mine has been weird for the longest time and it makes me so angry

 

this is the makings of a recall here

 

my dad 06 does this too it sticks a 250 when at idle and then works about 1000rpm . junk dodges these 3rd gens a falling apart now

 

Yeah I wish they would its starting to act funny again