1992 250hp looses rpms

[richd]

I have twin 250's, the port engine has a rebuilt powerhead with just a few hrs. on it. Runs great but then will lower rpms to about 1800 . After a few minutes rpms will usually come back to 4000 or so and then runs fine, and then this scenario repeats itself. Mechanic checked the carbs, fuel pumps,tps, and all sensors and he gives up. Mechanic says that when the rpms slow down, the timing shows dead on center. The engine isnt bogging down or sputtering, it will run just fine in neutral and then throw it into drive and it wont go faster than 1800rpm, and other times it is fine. This is the 2nd year mechanics are trying to fix this problem. Need help bad
Thanks
Rich

[boats.net]

Need to check all sensors at the time is drops to 1800 rpm's. Starting with crank and knock sensor. Check voltage going to ECU and all wiring and grounds. Check shift switch.

[richd]

Mechanic did all that and now tried switching the computer with the other engine. He said he cleaned all connections when doing so and that the problem is solved. I will sea test it on Tuesday, hope he's correct.
Thanks
Rich