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car jitters while driving

I have a 1999 Ford Explorer and I wanted to know why it jitters when I drive it?  I basically will be driving normally and then I can feel like it's going to turn off and then accelarates automatically.

Might be time for a new vehicle.
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You can take it to a repair shop and save a bundle. I would add fuel injection fluid to your gas tank. That fixed a similar problem I had.
Had similar behavior once with a cracked & wet distributor.  Felt like the clutch was slipping, then sputtered, almost stalling.  Since I had my foot on the gas, as soon as the sputtering stopped the car took off.

I suggest taking it to a mechanic and letting them diagnose.  I self-diagnosed the clutch going out...and ended up with a nice new clutch.  Then the mechanic called to tell me I needed a new rotor & plug wires while he was in there doing the clutch. (Head slapping moment.)
Jittering could be as simple as a miss.  If you haven't had a tune up or had bad gas recently I'd consider going that route.

Commented:
I had the same car...finally got rid of it after so many problem.  This same thing happened to me a number of times.  Once it needed a tune up with plugs and wires, second there was a bad o2 sensor or 2, third was a head gasket.  Finally we traded it in for something else.  But best of luck.
It might be the fuel filter.. they are usually easy to change, and pretty cheap to buy.
I guess I would be remiss if I didn't ask how much of a danger to you see in your unexpected acceleration? If it is getting uncontrolled, then go to a shop immediately. Have it towed if it poses a serious danger.
dhsindyRetired considering supplemental income.

Commented:
The problem could be any of the above.  Do you generally maintain the vehicle or just wait until runs bad?
Could be contaminated fuel. I filled up once just as the tanker had finished filling up the petrol stations tanks. For the next couple of days I had bouts of missfires followed by normal acceleration which got steadily more and more frequent. I checked the electrics myself as it was before the days of engine management systems but couldn't find anything wrong. Took it to the garage and they diagnosed water in the petrol, said it would normally lay at the bottom of the filling stations tanks, but because they had just filled up the water would have been all stirred up and probably pumped up into my car. They cleaned the system and filtered the fuel itself and replaced it, it was ok from then.
 
^ I had a PWC with a similar problem.  Long term storage got water into the fuel.  Used a fuel additive (made for marine use) to trap the water and make the gas usable.  Unfortunately, couldn't just dump the fuel and start over...nobody wanted to take it.  Had to burn it off.  Luckily was only a few gallons in the tank.  Topped off with good 91 ocatane and the fuel additive (Sta-bil brand).
Your coil may be dying, you can get a coil load tested at a garage.
Also run your engine at night in a dark place with the hood up & look for any visible Carbon Arc Tracking on any of the ignition components.
Also see:
http://www.sacskyranch.com/CarbonArcTracing.htm
One other thought which can cause this.
Did your tank get some diesel (or other) mistakenly added? (simply remove the gasoline cap & check for non gasoline odours)
If so, you will need to drain the tank + fuel lines, then refill with clean gasoline.