1999 Chrysler Concorde Engine runs rough, loud

Twice while highway driving, my engine sounded very loud (I thought it was the truck next to me.) It shuddered and seemed to lose acceleration. NO check engine light or other light went on either time. When I got to work, someone else drove it and had no problems. HOwever, the brake lights stay on. I drove it the whole weekend in town with no problems. The following Monday, the same thing happened. I brought it to a trusted mechanic (I hopel) He ran a diagnostic and he said everything came back normal. So far, they've replaced the fuel pump after determining the pressure was bleeding down. All the tie rods had to be replaced due to wear and the brake pads, rotors and calipers replaced due to wear as well. Now, it's the brake booster that's causing problems - an ascending "whining" when accelerating. I'm depressed. Any suggestions?