instrument cluster fluctuating

having problems in instrument cluster on 96 1ton diesel, coolant temp running up to 190 and falling off to 140,oil pressure 40 falling back off to 10,volt meter increasing up to about 15,and fuel mileage went from great(22to24) to sh**.Batteries seem to be holding good, almost 13v with volt meter,and luckily have another oil pressure gauge in truck that's telling tha truth,coming straight off motor.That leads be to believe this is electrical, duh,but computer not picking up trouble codes,could this be sensor problem,or alternator going out,has anyone had a problem like this? Would appreciate any input,thanks

1 would definately have alternator and battery tested. Try to have them test the ground system as well. A good tech can do this.

2 inspect and wiggle connections underhood.

3 Unfortunately some dashboard problems are under the dash and you have to pull out the clusters to look for shorts and test for good voltage

appreciate it