Underbody Shield problems of the 1998 Ford Escort

I have had near accidents relating to car fuel pump or transmission shops couldn't find any problems but the drivablity of my 98 Escort zx2 cost me more money in problem solving, 1,500 plus in recent repairs including emergency and repeat repairs and faulty repairs. My fuel economy is 160 miles per tank to almost 200 miles. No one can seem to find the problem(s). Large thunking in rear trunk (scary)when im taking off and reaching a steady speed. Take the car to major Ford dealerships, to no avail. Car doesn't always "go" from a complete stop in traffic,this has caused quite a scare as well. Thank you for helping and making note of my account. Debbie.