would have to post your engine serial number one reason Mercruiser did not have any new 454 in 2002. The 6,2 - 377 and the 8..1 - 496 - replaced it in 2001 along with the 502 --- ( black production side ) .

try swaping the relays and see if the problem follows the relay. if not check all your connection s from the key switch back.

Check voltage at pump and then check voltage in and out of relay.
You'll have to check it when the problem is happening

pull the fuel pump relay and look at the spade lugs of the connector,,see if they are over heated,also look at the bottom of the relay.merc went from thin pins to wider pins.

I just had this issue on my 502 MPI. Marina replaced the main power relay ($40 - easy to get to on top of the motor). Thought that fixed it until I took it out - hit a good sized cruiser wake and motor shut off. Turns out if was a 10 or 15 amp fuse (forget which) that had a fractional break in it (fuse was in the same area as the main power relay). Both items are super easy to check.

Check fuse holder also for corrosion and tightness of the fuse itself in the holder. Also check the power wires into the VST if you have one as well as the harness connector for it. If it winds up being the fuel pump in the VST, sourcing one here is much less expensive than OE the last time I looked ($80) https://speedncustommarine.com/