1999 Jeep Wrangler mileage: 120,000. When I drive my Jeep on long trips, after about a hundred and twenty miles, the oil pressure gauge starts acting erratically. The Jeep runs fine but the oil pressure gauge pulsates randomly between Zero and Forty. It only does this on the highway and after about 2 hours of driving. All other gauges seem OK it's just the oil pressure. This gauge drops causes the "check gauges" light to come on.
You either have a faulty gauge in the dash or internal engine oil pressure problems. To check this, connect a temporary mechanical gauge to the engine. Drive the car on the highway as you did when the problem occurred. Watch the mechanical gauge and if it does not fluctuate, then you know you have a faulty gauge or sending unit. If it does fluctuate, you have internal engine oil pressure problems.