All fluids is okay but when I'm driving the rpm needle go back and forth and then there's a hard stop and then the car shuts itself off. I turned my car off. Then istarted it up again it was fine but after driving 20 more miles the engine reduced power comes on. I wait another 20mins and it starts but it won't turnover. What could be the problem

Usually when the engine is "hunting" for the idle speed, the throttle position sensor or idle air control valve is faulty, or there is gunk in the throttle body, or a vac leak somewhere.
The computer controls the idle speed by using that control valve.
Is the check engine light on when the engine is running ?

No way to tell what happened with the high idle if you can't make it happen on demand.
The oil and engine lights would come on if the engine died while the key was still on so that was a normal event.
You might stop somewhere and have the computer scanned for trouble codes just in case the computer recorded anything.
The difference in octane should not be a big problem if you don't switch back and forth, after about 50 miles of driving the computer will adjust for the octane of the fuel.

this sounds like you need to put more coolent in the car if the needles moving back and forth on turns maby its low on fluid. after changing the water pump you could have a air bubble in the system that has worked its way out and now needs more fluid. i hope this helps

Take your vehicle to nearest autozone have them test .tell them your check engine lihht comes on and tell them your symptoms .the test is free and they may get a computer code telling if a sensor (which I suspect) is bad.please leve good feedback if you are satified with this solution.

Your car may need transmission fluid. Generally when there is a hard shift in gear, the transmission fluid is low. You may need to add a little transmission fluid.

same thing happens with my 2000 windstar, if you could take the instrument panel out, and unscrew the the back of the instrument panel that covers the circuit board and clean the circuit with a brush, pure bristle brush is best 'coz they're stiff and gets all the dust that accumulated on the circuit board. some dust particles contain conductive material and shorts the circuit board and cause the trouble you have, ever since i did that no more trouble. and to those who are planning of replacing your instrument panel should do this first. it'll save you money. by the way i am a mechanic

YOU NEED TO BLEED POWER STEERING SYSTEM BY HAVING LOW FLUID AIR ENTER THE SYSTEM.PARK CAR ON SOLID LEVEL GROUND SAFELY RAISE SUPPORT VECHICLE ON JACK STANDS.YOU CAN LEAVE CAR ON GROUND BUT TURNING STEERING WHEELS BACK AND FORTH WILL WEAR SPOT IN TIRES.TURN STEERING WHEEL TO THE LEFT WITH THE ENGINE OFF.CHECK POWER STEERING FLUID.ADD MORE FLUID IF LOW.MAKE SURE FLUID LEVEL ON COLD MARK ON DIP STICK.THE YOU CRANK CAR UP LET IDLE LITTLE HIGH FOR 15 SECONDS.THEN TURN OFF ENGINE.CHECK POWER STEERING FLUID LEVEL MAKE SURE ITS ON COLD MARK AT DIP STICK.THEN YOU START THE CAR UP WHILE IDLING TURN STEERING WHEEL IN BOTH DIRECTIONS SLOWLY TO THE STOPS DONT HOLD IN STOP TURN STEERING WHEEL LEFT AND RIGHT FOR SEVERAL TURNS.STOP ENGINE CHECK FLUID IF FLUID TAN STILL HAS AIR IN SYSTEM.LET CAR SET A LITTLE WHILE TRY THE BLEEDING PROCEDURE AGAIN.THERE IS A REASON POWER STEERING FLUID WAS LOW CHECK ALL POWER STEERING HOSES AT POWER STEERING PUMP TO THE RACK PINION ASSEMBLY.

rack and-or pinion... Good thing the 98 is a very good and popular windstar, its not as bad as it sounds.

What type of car???? could be something in the computer/engine control system. if its a late model have it checked out could get worse

Check the transmission fluid level and top up