engine question

[Country70]

i have twin 5.7L inboards that were replaced last year year so very low hours. had both outdrives completely serviced about a month ago. 2 weekends a go went down boat ran great. last weekend boat was really sluggish, topped out about 14 mph, ran for about 20 mins and all of the sudden it took off. got up to about 40mph, but as soon as I backed off the throttle it would do the same thing. changed the fuel filters last week and yesterday same thing happened. the only way i could get the boat to run was start from neutral with the starboard, and then the port engine, then after taking the both out of neutral I could go both together, but was still sluggish. Any ideas