engine surge on 1998 Chevrolet 3500 Pickup

engine surges at a stop(in drive) after driving at hiway speeds, it surges from 250 to 800 rpms. i've had the local shop check it, they tell it's not showing any codes, so they have no answers.

Your engine surge could be due to a low idle condition cause by a dirty throttle body. If cleaning the throttle body does not correct the condition you may need the help of a shop with a little bit better diagnostic capability.
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our 1999 gmc 3500 6.5 diesel duelly service truck was doing the same thing. it wound up being the fly wheel was cracked. we replaced the flywheel and everything went back to normal