2004 Ford Freestar - When accelerating and around 2200-2400 rpm, the engine will suddenly lose power and the rpm's drop immediately. I have to let off the accelerator totally and then slowly press down again. There is a squeel when I lose power, not loud but always accompanies the loss of power. I have replaced the ignition coil and found that the belt tensioner also need replacement. These have not fixed the problem.