I had the same symptom, like someone turning the key on and off really fast, randomly and only under load.
I switched the port ignition module (cutting out side) with the starboard one. Then the starboard engine started cutting out. Defective ignition module. Replaced it and it ran fine.

if shift interupter will stall when put in to gear not once under way. more then likly its in the fuel system but could be one of many things.....

first to check would be filters and line, tend to think cuda is right on that its in the carb need to listen to how the motor sounds just before it dies, is it starving for fuel before dieing or just cut out abruptly

could be dist advance but havnt seen that in a looong time and normaly dont think that would stall it just give no power


but ive been wrong before

Ok, heres a good one. Back in the old days when the igntion circuit wiring circuit was protected by a fuse, I had a engine that would run for 15-20 minutes at wot and then drop dead. Wouldnt start for 15 minutes, then started and ran fine for another 15 -20 minutes at wot. If you ran at less than wot, it would run all day. Fuel problem of course, right? Not. And I wont go into all the time and effort I spent troubleshooting it. Turned out it was the damn fuse that was getting hot enough to go open (like a semiconductor) only when the ignition load was maxed. New fuse, problem stopped. That one taught me to never ever assume to know what was going on based on the symptoms.

You need to check your fuel pressure, ignition, etc., and check for codes. There could be something easily located with a scan tool. The iac (should I even touch this) can slow the engine down on some engines. However, not this one. In 1999 all power reduction capabilities are off according to merc. The computer still can be at fault with no codes. You have twins, swap computers or any other suspect parts and test run. Just don't change them all at the same time.

So my question is, If your windhield wipers stop working, should you check the IAC?

yes

NO! Darn you ,Joe!

and if the radio stops working check the tps

PROBLEM FIXED!!!!!!!!!!!!!!!!! Thank you to all who gave there expert advice. The problem ended up being this. I guess I heard from someone that Formula or shall we say Mercruiser has had many problems in the past with the same problem that I had. IT was coming from the fuel pump. So instead of them fixing the problem all the did was get a work around and they put this workaround in boats that came from the factory. All the formula boats now have a fuel pump BOOSTER or some call it a FUEL PRIMER. IT is between the fuel tank and the fuel filter. For some reason with this particular engine they were getting this on/off reaction under load with the fuel pump that they were using. So what they did is just add this fuel primer or Booster and that solved the problem. So located the booster took it out and found out that the motor was stuck on it. So basically the engine does not work good on its own fuel pump it needs this booster to help it work. So I just purchased a new one and when it arrives I will install and finally enjoy my boat. Thanks again to all who give advice.

merc also with that fix had a problem with the debris from the fuel tank clogging the screen on the booster pump and they came up with a bulletin that has a new pre filter so the pump does not burn up,

it was primarily for the 496 when it first came out if you want I can find the bulletin for you