I had similar issues with my 454 (its older than your's 1987) i did everythign, i thought it was losing fuel also I would have dropping revs a sometimes just loose power for periods of time.
Nothing worked until I found out my timing was off 4 deg, new distributor cap, button, and new plugs and wires. while i was working on i found out the main wiring harness was loose and realt dirty inside i cleaned it out too. made a big difference in performance.

Took the boat to the shop. They found that the longer the boat ran the less voltage there was at the dash. They ran a new 10 gauge wire power wire to the dash and it seems to be fixed. I hope to test run it for myself Sunday......Thanks for the Ideas and suggestions.

Good deal, glad you got it figured out. Let us know how she works out...

Test drove today and I believe its fixed. A very expensive piece of wire from battery shut off to the key switch....just glad its fixed......Now where did summer go?