Yesterday my 5.7Mag MPI started to develop an erratic miss about every 5 to 10 seconds.It didn't seem like it was losing 1 or 2 cyl, it was like it was losing all cyl for a split second. I noticed the tach would jump from 3000 (RPM I was running) to about 4000 or 5000 in that same split second and then everything would return to normal for another 5 to 10 seconds. I limped it to shore and put it in neutral, gave it some throttle, and it acted like the choke (I know, it doesn't have one)was closed. Shut it down and checked some simple things but didn't see anything obvious. Limped it to the nearest marina and called some friends to give us a ride to our truck and trailer.
It seems it is ign related, however I am not familiar with this system. I am a ASE cert auto tech with good knowledge in auto ign and fuel injection systems. I just wanted to see if anyone has had a similar problem before I get out the manual, meter and test light.

2000 5.7 Mag MPI/Bravo with 270 hrs

That might be your best bet to get out the manual, check for loose connections, etc, and check the computer for error codes.

Pull the distributor cap and look underneath. My bet is there is corrosion causing it to cross fire. This makes the tach jump around also. The jumping tach can also be plug wires.

Good luck and post your results,

Ron