1997 chevy k3500 4 speed auto issue

[Whyzedtwofiveoh]

I have a 1997 chevy k3500 that has been sitting for about 4 years outdoors...i bought it from a mechanic who installed a new transmission for the owner who never picked it up, it has the 4 speed auto transmission. The issue is that at from 60kmh onwards the transmission shifts up and down quickly under heavy acceleration not allowing me to climb past 70kmh. Under smooth acceleration it will climb to 100kmh. 0-60kmh under heavy acceleration is fine with a solid shift. The engine has a minor miss/sputter in it, im not sure if this could be part of the problem, im really unsure were to look to troubleshoot the problem. Any help is appreciated

 

[96Z71ECSB]

Fix the engine miss first, then go from there.

 

[454ss]

mine did that same thing when 4th was going out cause the o-rings on the input shaft were dried up/shrunk and it wasnt holding pressure, that was on the original trans with about 180,000 on it. maybe from being a fresh trans with no miles and sitting so long its the same issue. could try some lucas but imo thats not a proper/permanent fix, the mechanic ya bought it from doesnt have any ideas?