I had a similar-ish problem with mine for a while too.

Mine would be sluggish to start with, then drop out, then come good.
I checked the transducer doodad in the transfer case.

Turned out to be 3 screws on the back side of the speedo not screwed in tightly and making an intermittent connection electrically for the speedo.

There are 3 (I think)flat head screws, maybe brass or copper that hod speedo tight to the housing, but also make the connection.

I'd be checking them first.