Suburban turning off???

Hi,
I've a Suburban 1990 3/4ton 5.7 but I'm going through some problems.
Sometimes whem I'm driving the sound system goes off, and so goes my car.
Moreover, the ceiling light doesn't goes on when I open the door, any of them. Otherwise I don't have any problems with the ignition.
There are eletric sparks on the engine, the fuel pomp has been changed so was the fuel filter, and the car passed through reviews, but nothing wrong was found. Even the alarm system was verified and so was the eletric part.
Then, I'm wondering what can it be and what can I do to solve the problems.
Thank you!

Start with the basics..check the battery cables and connections. A volt drop test should show less than .2 volts across the connections.
A volt drop test from battery negative to engine should be lass than .5 volts