I've been having problems with my fuel system regularly since I've owned this car. I have had the in-tank and main fuel pump replaced in the last 25K. Since my prepump has failed once before, I am familiar with those symptoms, and the current symptoms are similar. When the tank is half empty, the main pump gets very loud and the vehicle exhibits the symptoms of fuel starvation (though it does not seem to be 100% when the tank is full, either). I asked a mechanic (I'm never going back to!) to diagnose what the problem was. He tested the pre pump, which is fine, but never told me what the cause of my problem is. This is an old car with mega miles, and I frequently use the cheapest gas aroundnd let the tank run low.
My question(s):
1) Because of recurring fuel system concerns, should I suspect contamination? How do I test that?

2) What else could be causing the symtoms described above?

Any help is mucho appreciated!