1999 Chevy Venture want go to 3rd

Just curious if this automatically means I need a new transmission. It switches to 1st and 2nd fine. After 2nd I start hearing this humming sound, and it want go to 3rd. I went like 50mph and never
switched, and noticed when I let off the gas I slowed down really fast and I never applied the brakes.
I have to say that I have been waiting for something like this to happen. For the past 4 months when It would switch to 3rd it would kinda jump forward a little, like it was stuck or something. Say right after It did it I had to stop at a light. It would then do it in 1st, 2nd and 3rd. It would continue to do it until I turned the car off, and restarted. Then it would go weeks sometimes without doing it at all.
I took it to my mechanic (who by the way is very good, awesome and trusting man) he could not get any codes, of course when I took it, it wasn't doing it at the time anyway.
I learned early though that once it did it in 3rd I would just pull over turn the ignition off and I'd be fine. It always worked. I've been doing that for months now because it never does it during the day when my mechanic is open. Always at night when it's cold.
Tonight though when I left home It just would not go into 3rd at all. I checked the fluids and no problem there. So, my question is what could this be? Am I going to need a new transmission? 1st and 2nd work fine....What could be wrong with 3rd? I read somewhere that shift solenoid could be my problem. I also read that i could remove the PCM fuse and reset the code if indeed it was the shift solenoid and drive it to see if it fixed the problem. If so, then what? Do I still take it to my mechanic? PLEASE ADVISE!!!! PLEASE!!!
I'm one of those worrying folks who lets stuff like this drive me crazy. It's 2:30am and want know nothing for hours. It's driving me mad.
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This needs to be scanned with a full function trans scanner, the tech can command shifts and confirm solenoid operation and slippage.

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