SR5 Cluster swap into a Base 4cyl T-100

• I have done this swap on my 1992 pickup without any issues.
  
  So I figured , what the hell ?
  
  Let me give it a shot on my T-100, i found a SR5 cluster rather cheap , made sure to get the proper oil pressure sender and adjusted the mileage .
  
  Boom works perfectly , One issue ....
  
  Tachometer bounces all over the place when the truck is idling , when you are under load it seems pretty stable but not perfect .
  
  I realize that this cluster was meant for a 6cyl engine and not a 4cyl.
  
  I attempted to adjust the pot on the back of the tach , setting it to something similar that i did on my 1992 pickup 63.8K ohms
  
  Any ideas ? Anyone else come across this ?
  
  Thanks in advance