5.0 to 5.7 with RPM issues after change over

I recently had my busted 5.0 block on my 1999 FourWinns 238 Vista switched to a new 5.7 replacement block. All other systems on the boat were kept from the original. ECM was sent off to be reflashed and now that the boat is back together I cannot get the boat to run past 2500 RPMs. Boat will run fine at 2400 and below but as soon as you go past 2500 the RPMs will bottom out and bounce back and forth between 2500 and below 1000. First was told that the timing was not set properly on new block so had a local marine tech come out, who used program on his PC to properly set timing. Just went down last night to test out after work was done about 10 days ago and had the same issue, other than now it bounces down so low it almost stalls out. I have seen post talking about blocked exhaust, fuel vapor lock, ECM locked in slow mode, fuel pressure, fuel water separator issues, fuel cooler (not sure I have one). I did not put new block in myself had friend of a friend do this and motor came from well-known boat motor replacement company. They are ones that suggested that timing was off and that is why it would not run. They were also the ones that sent my ECM off to be reflashed. I need help almost $5500 in the hole now without a running boat. Thanks in advance to help, Niner

I recently had my busted 5.0 block

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Ayuh,.... What killed the old motor,..??

That in itself may be a clue to what's goin' on now,...

Having owned a 238 vista, I want to point out that your rpm issue sounds -exactly- like what mine did right before the fuel pump failed.

mine was a 96, but the fuel pump did slowly fail. And when it did, the boat wouldn’t start. I replaced it (super easy) and never had the problem you described ever again.

I know you are looking at apples and I just threw an orange. But check your fuel pressure before you start messing with anything else.