1988 mrrcruiser 5.7. Rpms inconsistent

I have a 1988 Malibi Skier, it has a mercruiser 5.7l inboard. I've been struggling with certain issues since last season and I need help asap. My issue is that the boat will fire right up on land and run perfect, but when I drop it in the pond, it will fire up and run fine going fast, 30-40mph but once you slow down it will sputter and stall out on take off. If I try to hold the throttle open and turn the key it will fire but the rpms will raise them stall. Sometimes they only hit 2000, and sometimes they will reach between 3 and 4. I continually take the fuel filter out and clean it, or replace even though it is not dirty. After this the boat will start and hit 3k rpms and stay there but it doesn't last more than 15min. Any ideas? The motor has just under 900 hours and I have done the following this year
New Mr. COOL Transmission Cooler
New OEM fuel pump
Drained Fuel and pulled aluminum tank, soaked and cleaned
New fuel filter and water separator
Carb soaked, cleaned and sprayed with compressed air (Rochester Quadrajet)
Points wires, distributor cap
New plugs but I am replacing today and doing a compression test. Sorry this is so long does anyone have and ideas

1988 did NOT have points.

So maybe clarify that.....should be a Thunderbolt IV

Back to your issue

Sounds fuel related, anti syphone at has tank and or gas tank tube clogged or gas tank vent clogged

kghost said:

1988 did NOT have points.

So maybe clarify that.....should be a Thunderbolt IV

Back to your issue

Sounds fuel related, anti syphone at has tank and or gas tank tube clogged or gas tank vent clogged

Click to expand...

I'm almost positive it's fuel related. Anti Syphon is clean, Breather hose is clean. I'm stumped