Boat is off 10mph all of a sudden

[80hpmerc]

Hi all, my 1982 delta craft with a 1982 80hp mercury has suddenly stop getting top speed. I'm off 10mph. I used to be able to get 35mph no problem. Take off real fast and cruise beautiful. But now when I took it out it would stall when i go full throttle. I have to gradually increase speed and I'm maxing out at 25 mph. Also too while going full throttle it will do short bursts of power wanting to take off but then stops right away. Probably all within 3 seconds. Like I said very short bursts of "proper" power. My volts hold fine at 14. They flicker briefly while I engage the trim hut always go back to 14 and Holds. I'm completely stump. What I've checked and done so far

1. Brand new gas tanks
2. Brand new fuel line and primer bulb
3. Took out fuel filter and cleaned.
4. Inspected pump and seems to function proper. Meaning it springs back in place fine. (Did find a small hairline crack in the unit that the filter and pump sit in and had it fixed as I cannot find one around me and mercury doesn't make them anymore or carry it and cant get in aftermarket parts either.
5. Had the throttle lever and spark screw replaced as it was cracked and would stick sometimes.
6. Brand new spark plugs
7. Carbs look fairly clean. Ran seafoam after this issue started and still no good.

Any suggestions or additional questions please shoot away. I'm stumped and cannot afford the ridiculous repair Bill's associated.
BOAT=Bust Out Another Thousand

 

[Faztbullet]

Need to check spark as sound like switchbox has dropped a cylinder

 

[Chris1956]

Take a compression test. If a cylinder is scored, power will be down. Take a spark test. Maybe you have intermittent spark on a cylinder. That would account for bursts of power.

Not sure what a spark screw or throttle lever are in your context.