A442F trans shifting up and down erratically near full throttle (1 Viewer)

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Hi, my fairly stock 80 (1996 1HD-FT HDJ81) has about 380k kms. Over the last year or so it has started giving me this problem: when I'm in top gear and wanting to speed up, as I press on the pedal it gets to a point where it shifts down and up erratically, as if it's not sure what gear to be in. Which makes overtaking a total pain.
Fluid probably was changed 80k kms ago, before I owned it. I thought of flushing it, but it looks good as new. It might be slightly over-full, not sure if that has anything to do with it.
Not sure if it's my imagination, but it seems worse when hot.
I played with the line pressure cable but it didn't help, just made shifting more jerky.

Anyone got any ideas?
 
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If it gets worse as the fluid gets hot, then that would potentially agree with it being slightly over-full. (the fluid expanding when hot would make it even more over-full).

Does the FSM have a diagnostics section for the transmission? Maybe go through it and see what symptoms have transmission fluid overfilled as a cause.

Also, does it happen when you stomp on it or only when slowly increasing the throttle? If its the latter it would indicate an issue with the hysteresis circuit in the valvebody. I have no clue how hydraulic computers do hysteresis, but I could see too much backpressure in a drain (from overfilling) as a cause.
 
I'd start by first getting out my factory service manual so i could check/test the shift solenoids located inside the transmission pan. On transmissions with high miles they can start to fail causing shifting problems.
 

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