99 Auto LX, 242500, video attached in link.
https://photos.app.goo.gl/rucdfBVRDGyWYLhD9
This only happens from 3-4 when under load getting on the highway. It would normally shift around 5500 but now itt just climbs almost to the limiter, triggers the cel 5 times, then upshifts like it should and the car acts like nothing happened. No misfires, lights, smells, or weird shifting.
It's super bizarre.
This started happening after I drained the old fluid and added new. (To the correct level.) No flushes or anything, I'm not about blowing debris around the valve body.
The car drives flawlessly 99% of the time, it just does this one weird thing in this specific instance. I'd like to get it resolved for peace of mind.
One thing to note is it shifts completely normally 1-4 if I'm not flooring it. I can floor it 1-3 and then go easy until 4th and it'll be normal, it's just while flooring in 3rd at ~60mph. Hence why I suspect either shift solenoid or dirty fluid

 

I'd really like any sort of advice. I need to get this sorted asap for as little money as possible. Perhaps I need to change the fluid again and again until it's all new?