Try taking the dash apart to get to the plug on the back of the cluster. Clean that plug and make sure its not moving around.
To get dash apart:
1) Pull the defrost bezel at the top of the dash straight up (it is held on by clips).
2) Remove the Radio/Air conditioner control bezel by removing the two screws at the top and one behind where the ashtray would be.
3) Pull straight out on the bezel
4) Remove two screws that hold kickboard panel (under the steering wheel) and remove panel.
5) I think it is 5 screws from here to remove the entire cluster. (I only went as far as removing the clear plastic protector)
1998 Jeep Wrangler Sport 4.0: Hardtop, 4" Procomp Lift, 1"BL, 35" MTR's, Super D30(Chromoly)-Super D44(Chomoly), 4.88 Gears, ARB Lockers