96 Express with strange problem

Boy I've got a wierd one for you guys. I've got a 96 Express Van (G1500) with a 305 V8 and 130K Miles. I first noticed this problem when cruising at 60 on the highway (on cruise control). I topped a very slight incline
into a slight downhill segment of the road. As the cruise went from accel through float, and into decell, there was a single jerk. It was rather abrupt and I actually thought I had hit something. For the next hour I tried to duplicate this (chuggle, bobble...). I could duplicate it under the same conditions (cruise, over 55mph, hill, slight downgrade). I was finally able to duplicate without cruise control, but all of the other conditions had to be the same. It would only do it with a very slight movement of the pedal. My first thought was broken motor mount. I checked them over, and found a disentegrated trans mount. Great, I thought, fixed. Bzzzzzzt, wrong. It won't do it when I accellerate hard, and just release the throttle. It won't do it unless I am on a slight downgrade, steep downgrade won't do it. Oh, no codes either. Any Ideas?
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i'm not sure what cruis controle system you have. could be a sticky speedo cable. if your sure its not the cruis controle you could look at your vacume advance or the mechincal advance return springs under the cap, could be a carb issue if it has one, could also be a dirty or clogged injector.... no way to tell from here.
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dirtdude wrote:

    No 1996 GM Vehicles have Carburetors. They are all Fuel Injected, Including Medium Duty Trucks. Charles
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KENG wrote:

    Has the transmission had a fluid & filter change? If not it is past the service interval for such. Charles
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ATF and filter have been changed 3 times, once at the first oil change to Full Synthetic, then once at about 65K, then again when I replaced the trans mount.
KenG
Charles Bendig wrote:

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