Probably the ESS, engine speed sensor. Down by the balancer on the front of the motor. It should be about a playing card thickness from the balancer. Also check the wires leading to it, they can be worn through around the water pump.
Fuel gauge is a common problem, the sending unit's aren't very reliable.
'96 2500, CTD, 4X4 flatbed, Isspro gauges, Skyjacker D-25's, ThurenFab track bar, DSS, HD steering.
Motor; BHAF, #100 fuel plate, 5X.012 90hp injectors, Pacbrake 4kGSK, Loose starwheel. 306hp/743tq before injectors.
Exhaust; Schwitzer S300G, 4" downpipe, single 5" turnout stack.
Trans; Dunrite VB/TC, Intelligent Engineering billet input shaft, Burt Brown rebuild/upgrade.