Very strange problem. My 1995 Grand Voyager bucks and rpm's surge. What can this be?

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I have a very strange problem with my 1995 Grand Voyager. It will buck and rpm's surge from 2 to 3 ½ after driving for about 10 minutes at 60 to 70 mph. It will continue to buck much worse as I continue to drive. The strange part of all this is as I'm driving I'll put the car in neutral and rev the engine until the rpm's hit 4 and then wait for it come down to 2 and then put the car back in drive. The problem miraculously disappears until my next trip the next day. What can this be? If it were the tranny, I would think by revving the engine in neutral wouldn't fix it. I'm puzzled!