Intermittent shuddering while driving

My folks have a 99 Intrepid. While driving this weekend, they experienced occasional shuddering. when they got home my dad searched the motor for loose connections only to find nothing. So I checked for codes using the ignition on off on method and came up with P 1684 and P 1391. Anybody know what these codes are, and would they have anything to do with the shuddering? My dad says it feels like the motor is cutting out or missing.
Any help would be appreciated. Thanks

P1684- The battery has been disconnected within the last 50 starts. This code doesn't set a MIL (malfunction indicator lamp), it only means that the battery has been disconnected recently. P1391-Intermittent Loss of CMP or CKP (Loss of Cam Position Sensor or Crank Position Sensor has occured.) Start by checking the connections of the sensors, If it has the 2.7 V6, the cam sensor is located on the front of the driver's side cylinder head just under the valve cover. Make sure the connection is good. The crank sensor is mounted on the passenger side of the car on the transmission. If the crank is going bad, the car will start shuddering like it's about to cut off.

I know this post is old, but since then, my dad has taken his Intrepid to have both sensors replaced, only to have the shuddering reoccur from time to time. Could there be any other possibilities, i.e. computer, plugs, injectors... He's to the point of trading the thing in. It's a nice car with low miles, I'd just hate to see him get rid of it if ti turned out to be something simple. He checked for codes, but the codes that came up before the sensors were replaced are now gone. Any Ideas? thanks.

BTW, it is a 2.7 if it helps at all. Thanks

try having the fuel system cleaned out really good, for example, what I use a something Lucas Fuel Treament & Upper Cylinder Lubricant commonly found at Advance Auto Parts, stuff is 3.99 a bottle but it's concentrated, and powerful to clean out any garbage in the fuel system/injectors.

Well, I tried the Lucas fuel treatment, figuring it would be a cheap fix. No luck. thanks for the advice though. On local drives it acts fine. It's just when they travel for a distance (for an hour or more). Could it be a faulty computer. My Dad says that the mechanic is looking for a used computer to try in the car to see if that helps. I think that by the time he plugs all that money into trying to figure what's wrong, it'd be cheaper to trade it in!!! Lol.

may have a faulty fuel pump

My daughter's Explorer was doing the same thing. They took it into the dealer and had the fuel filter replaced. Took care of the problem. It makes sense if the car is starving for fuel while under load, like high speeds.