Hi MrFent. Welcome to the forum. This is going to be impossible to diagnose without being right there, but I've had a bunch of these cars, (still have three of them) and can give you some things to check to get you started.
By "jolting", do you mean the engine is sputtering or something in the steering or suspension is shifting? Based on all of the symptoms, I'm guessing you have an engine problem.
Your engine management computer is bolted to the front driver's side of the air cleaner housing. There's a vacuum transducer on the front of it. Pinch that hose closed with a small c-clamp, then drive it to see if the problem goes away. You can also remove the hose and plug it but you must also put a cap on the transducer port. I can't explain why, but one of my cars developed a surging problem only at highway speeds and unplugging that hose reduced the severity of the problem, and capping the port eliminated the surging completely. That was many years ago before I understood how that system worked so I never did solve it. Instead, I installed an older style ignition module and an older distributor with the timing advance mechanisms built in. Since there was still some surging until the vacuum port was capped, it's possible the transducer was reacting to wind blowing into it. That would cause ignition timing to vary resulting in that surging feeling.
Another problem I had with the same car was a worn mounting hole in the front cross member where the lower control arm bolted to. When turning slightly at higher speeds, such as when changing lanes, the control arm shifted position causing that tire to sort of fold under the car. The severe misalignment caused tire scrubbing and a loss of speed. I could imagine someone calling that sensation "jolting". That can be a real hard problem to find because it won't show up during a routine inspection. The biggest clue when this happens is the steering wheel will shift position. When you steer the other way for a moment, the suspension will straighten out and it will feel like that corner of the car raises up a bunch.
Another thing you might consider is a defective ignition coil. I just solved this problem on my '88 Grand Caravan, and some of the symptoms were exactly like you described. The problem actually started 11 months ago, but it acted up so seldom that it was impossible to troubleshoot except by throwing new parts at it. That is the least effective, most costly way to diagnose a problem but it was my only option. At times the engine would die completely and not restart for one to two minutes, but every time I checked, it had spark and fuel pressure. Part of the problem in finding this is some of the symptoms did not match what typically happens when coils fail. In particular, on hot days, the engine would die at anything over 3/4 throttle and continue running when I let off the gas a little. At other times there was a bad misfire and shudder which definitely could be described as "jolting". One of the clues I overlooked was the problem got worse after I replaced the spark plugs. I widened the gap a little which meant the coil had to work harder to fire them. Within a few days, the engine died completely on the side of the road and never restarted. Finally I caught it having no spark. The new coil solved all of those weird problems. A lot of what you described sounds like the same symptoms I was having.
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Wednesday, October 20th, 2010 AT 1:19 AM