He may be having the same problem I'm having that has been addressed in another thread where it seems the culprit is the "lead free solder" used on the contact points on 2 chips.
Lead free solder develops hair line cracks and loses it's conductivity.
I watched a guy on Youtube "reflow" his contact points and fixed his problem.
I actually had to do "reflows" on my old "FAT" Ps3 and a big screen TV.
The TV is still going good. The PS3 well... the third time I had to bury it.
I hooked my Diablo up for the first time today.
I'm enjoying the ability now to see my RPM's lol.
I had NO idea I was doing 1900 rpm @ 70mph.
I even created a thread asking about hooking up aftermarket tachs - which, I'm still going to do because, I'm not satisfied with what the Diablo has for "tachs".
*ETA*
Let me clarify, the "2 chips", are chips on the back side of the tach IN your cluster.
You have to remove the entire cluster (after taking apart the dash area) to fix this.
Here is the YT link:
https://www.youtube.com/watch?v=ufQ1t4eOwkM
You can also type in "Fix for DODGE RAM tach" into the YT search field.