Bob, Sounds to me like your main cartridge fuel filter is clogging up, that would kill top end power as you describe. Cool running, would definitely add smoke to your exhaust and reduce fuel mileage. You should verify the accuracy of your gauge (borrow an infrared thermometer and point it at the thermostat housing) as it seems like three thermostats should have done the trick. One possibility is massive corrosion inside the thermostat housing is allowing some coolant to bypass the thermostat control. regards,
Jim Friesen Phoenix AZ 79 300SD, 264 K miles 98 ML 320, 146 K miles
