When you changed your in tank pump, were you sure you got the polarity right ? If your main pump is making a lot of noise the pre pump may be running backwards and the pumps would then be fighting each other. I had this happen once because the replacement pre pump was not marked + and - so I hooked the wires up as they came off the old pump. It took me a long time to figure out what the problem was, but it sounds very similiar to what you are experiencing. I disconnected the pre pump and the car ran much much better with just the main pump only. That is how I figured it out. Unfortunately, if that is your problem you will have to remove the pre pump all over again and switch the wires. You can check pump rotation from the bottom of the pump, there should be an arrow and the impeller should be visible. Hope this helps.