What Causes Jerking Between 45mph And 60mph In Gmc Sierra 1500 5.7 2WD

I bought my 1997 Sierra 1500 5.7 Vortec about 6 months ago. I love the truck and plan on keeping it for a very long time as it looks like new with only 74k original miles on it. HOWEVER, problems started when I noticed the check engine light come on. Took it to the dealership and the code read O2 sensor problems. No big deal right? Well they also found that I was leaking fluid from the intake manifold due to a faulty gasket. Looked it up online and found that this is a common problem. Since I knew this would involve stripping the motor down to the intake manifold I elected to replace all critical parts so I would not have to worry about them for a while (intake manifold gaskets, valve cover gaskets, spider injectors, serpentine belt, O2 sensors, water pump, pcv valve, plugs, wires, distributor, alternator, thermostat). I also had some brake work done (master cylinder etc.) Thinking I was done with major maintenance on the truck for several thousand miles I drove away from the dealership. Then it happened…..and has been happening for 2 days. Between 45 and 60 MPH with no hard or acceleration or braking the truck “jerks” intermitantly almost as if the tranny is slipping, but it seems to be in-between gears. This DID NOT happen before I took it in for service!!! Other than that the truck runs great. It is back at the dealership and they are currently at a loss at what it could be. My online research suggest anything from faulty or mis-adjusted brake light switch, faulty spider injector, to torque converter. Please send your thoughts/ suggestions to help stop the bleeding at the dealership!!!! Thanks. ZEP

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