Loss of electrical power - 89 Burb

For unknown reason my 89 1/2 ton burb will loss electrical power and it will stall out. I can jump it, starts right up, but then as soon as I put it in gear or roll up pwr window, it immediatly dies again. Then I have to jump it again and put the vehicle in drive before disconnecting the jumper cables from donor vehicle and I am able to get home. The lights will flicker a little if I slow down at all. When I shut off the engine, no pwr at all, then a few seconds later an interior light might come on. This has total electrical power loss as happened twice in the last few months.
I am thinking bad alternator, cables or battery. So tonight I took out the alternater, but the internal lights were still on inside the vehicle. I hit the pwr window switch and the lights all go out. :screwy::screwy: Will a grenade fix this.
thanks in advance for any suggestions. I am testing the alternater and batter anyway .:screwy:

Check all the grounds and battery cable connections.

The battery posts looked clean from the outside. I took off the cables from the battery and it was full of gunk. Cleaned it up with a wire brush and baking powder and haven't had a problem since. thanks too Maxwedge.