Mercruiser problem

I have a 1997 Mariah 252 Shabah with a 7.4 Mercruiser and a Bravo3 drive. My problem started the last 2 times I've taken the boat out. It will run fine for about an hour to and hour and a half and then it will start shutting down and restarting quickly. Its like I'm turning the ignition key off and on real quick. When it does it I've noticed that all the guages drop to zero and then jump right back up again. It only does it when I am above 2500 RPM. The batteries were 4 years old so I replaced both of them and checked all the wiring that I could looking for a loose connection or ground, but could not find anything wrong anywhere. The last time I took it out, after about 1 1/2 hours of running great it started it again. This time though after doing this for 20 minutes or so, the engine just died like I had turned it off. The battery was completly dead. (brand new battery) I switched to the other battery and restarted and made it back to the dock. But as long as I kept it under 2500 RPM it was OK. I was running on a fresh tank of fuel.

Sounds like a loose/bad wire. Is your battery definatly dead or is the connection to the battery dead??

Sounds like your Altenator might be bad and you are running on the battery when the voltage drops low enough the electronic ignition will cut out check the voltage at the battery when the motor is running there should be 13.5 to 14 volts. if not have the Alt. checked

I've had a similar issue.. Not exactly. I had issues with the engine cutting over 3500RPM's. My gauges would drop to nothing also. I thought it might be electrical....Turned out my fuel filter (not separater) was either clogged or the nozzle on it was creating vacuum that was starving my engine of fuel. I replaced it and the problem went away. This was after 3 trips to a crappy service dept that couldn't fix it and I finally got fed up and called the factory... they pointed me in the right direction and problem solved...without the service dept. Took 10 minutes to fix and it's a cheap part. Might want to check it. When was the last time you replaced the filter?

I replaced the fuel/water seperator filter a month ago or so but not the fuel filter. I will replace that asap. I was also thinking about the alternator. I was going to hook up my hose tonight and run the engine and unhook the batteries and see if it keeps running or not. If it dies, then I know that its running on the battery and that its the alternator.
Thanks for your suggestions.

NEVER EVER disconnect the battery on a computer controlled engine when it is running. You run a high risk of frying your computer.

If you want to check your charging sytem it is going to take a volt meter of some kind. Look at the voltage before starting the motor, then start it and see if it rises. Load the charging system down by turning everything you possible can on, and again check the voltage. You may need to bring the RPM's up to about 1000 to get an accurate reading. You should see about 13-14 volts with the engine running. If not you may have a bad connection from the alternator or a circuit breaker has popped, or simply a bad voltage regulator or alternator.

Thanks for your advice on not disconnecting the battery. I stopped by my local marine dealer and he told me the same thing. I pulled the alternator off and took it to AutoZone to have them testy it but they had no idea how to hook it up to the tester since it is a marine alternator and not out of a car. I am going to take it to my marine shop tomorrow and have them test it.
Thanks again for your help.