Having trouble with my wife's 745 turbo wagon....When running at 50-60 mph it will lose power,start chugging, then run only at about 20 mph. It doesn't die but won't accel over 20 mph. When you shut off the key it will restart and then run fine. It only does it a couple times a month. It doesn't seem to matter if the gas tank is full or less. Or when it's hold or cold..........We had it in 3 different shops but it never acted up when they have had it. They have replaced the main fuel pump, the air mass sensor, the fuel pump relay, fuel filter, vacuum hoses, turbo hoses,ignition control module, battery, main computer, cleaned the flame trap, checked grounds etc. You can hear the fuel pump running in the tank however, it still is the original (164,000 miles) The main fuel pump however has "whinned" since it has been replaced. It shows no service codes and they say the plug wires are not misfiring. It's got us puzzeled..............any ideas?..................thanks........