Engine runs out pf power on freeway

The transmission was just replaced with a New one,thought the neutral switch was bad.Same issue presists,lost engine power when accelerating.Dash still on Rpms up at first but seems like engine lugs down.

Also we checked the fuel pump,it checks out okay,however the fuel fliter was totally clogged.could there be sediment in lines?

What was the concern prior to replacing the transmission? Was there a total lack of engagement? What kind of diagnostic testing was performed? When the transmission was replaced, was the transmission cooling lines and coolers flushed thoroughly?

When the transmission "runs out of power", does the engine rev freely, or is it choked / bogging down?