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I have a 1994 GMC Sierra 1500 2x4 pickup.  Basically this is whats happening.  I was driving one day and noticed that everytime i let off the gas it would down shift for some reason then when i hit the gas it would jump back in gear and keep going.  I was alert, but didnt pay to much attention.  About 2 days later i was on the highway when all of a sudden i hit the gas and got to 35 mph and it felt like it went into neutral.  Id let off and get back to 35 mph and it would be drivable but as soon as i got to 35 it would go in neutral.  I got it home that night after being towed.  I drove it up the road and it did the same thing.  I came home and bought a new transmission filter and gasket and fluid.  Changed it all and took it for a ride.  I can now get to 45 mph then it goes into neutral.  Its a automatic transmission.  It shifts correctly at 25 mph but it feel like when it needs to be shifting at 35  it wont.  2nd gear rides out till 45 mph now and then goes in neutral.  I was wondering if maybe it was a sensor and something small.  Dont really want to spend 1000$ on another transmission   Please reply with any thoughts you may have
its a 4.3L vortec v6 with a 4l60e automatic trans