Hey All, 1988 GMC C3500 RCSW. Truck has 91KMS 5.7L TH400. Recent tune up; plugs, wires, cap, distributor. Truck runs ok but recently have noticed when driving at constant speed the Transmission hesitates/surges... I think. Into second gear at 40KMS (+/-25MPH) the truck RPM jumps without shifting. I've got a Snappy scanner (MT2500) and the increase is consistent at 300RPM increase/decrease. It eventually shifts and the engine speed evens out. At higher speeds there is no issue at all, highway and pushing it, it runs well and shifts fine. Only happens at slower speeds. No noticeable shift noise or delay just the fluctuating RPM. It's mostly a nuisance. When I got the truck last year I changed the fluids and gasket but have had no issues until recently. Heard the TH400 was bulletproof and really don't want to blame it. Any thoughts? TIA.