Electrical Problem

[95twodoor]

Hello, I have a 1995 2 Door Z-71. Recently I put in a new motor, new alternator, new battery, new water pump, belt, etc etc. After driving for around 100 miles breaking in the new motor, I am having electric issues. When I start the Tahoe, the alternator shows about 13 volt charge. After driving for a few minutes, the alternator shows around 9 volts of charge. At that point the rpm's start jumping around. Like from 800 rpm to 1300 rpm and back down. The engine shakes, stutters and dies. At that point it will not start for about 30 seconds. After it sits, it starts fine, but it does this all over again. I had the alternator and battery checked, they were both good, and I replaced them anyway. So same situation with 2 different known good batteries and 2 different known good alternators. The second one is even a high output but has the same result. I don't understand why this would happen after driving for a while. It seems like an issues like this would have started as soon as the truck was finished being rebuilt. Any help will be appreciated.

Thanks

 

[95TwinTT]

When you say a new engine, are we using the exact same engine that came out?

Are the engine control components the old ones from the previous engine, like EGR, MAF, MAP, etc, sensors?

Does the voltage slowly drift down from 13 to 9 volts or does it go down quickly after so many minutes.

Were any wiring connections replaced or any wires connected that did not have the stock connectors?

Are you using the stock ECM?