Did you swap the fuse card or just the relays? Problem is female socket in card. The EPO switch has been known to go open intermittently, causing shut down or weird codes. Jumper to test or replace the emergency power switch.
Put he truck up on jacks, engage the throttle to run steady. With motor running and wheel spinning, GENTLY wiggle wires at VM, cards, relay board. VM plugs are bad on 7400's for poor connections in plugs. Seen this on brand new trucks as well. After working on these for 10 years, then working on all other brands now, I realize how poorly designed these Raymond 7400's are.