I am running a 3pk with 2.4 FASST. I am losing connection after 40 Yards. New rx battery 1 charge, TX batt reading 10.3. Any ideas?

My first thought would be to check the receiver antenna for any cuts or breaks, especially near where it exits the case. beyond that I'd be stumped.

Maybe Interference?

interference with 2.4 ghz?

From motor ie noise

Interference is a reasonable possibility. 2.4 ghz band is used for everything electronic under the sun including older and low-end cordless phones.
Try cycling that new receiver pack a few times and see if that helps

Which rx - the one with the extra internal antenna (603FS) or without (603FF)?