Hmmm. Per your other post, I can think of a few other things but the vehicle speed thing adds some confusion. I should have added that a leaking heater core would leave an oily residue on the windshield and dash.
Can you stop and pop the hood open when this happens? For the smoke to be that bad, it would seem it is more than just an oil leak. I would start by looking for oil running along the back of the engine and dripping onto the exhaust manifold. There will be a black crusty residue if oil is dripping onto exhaust parts after they cool down. It sounds like the oil is puddling somewhere, then running off under the right conditions.
A totally different thing to look for is the generator / power steering belt. If there is no smoke when the engine is idling, have a helper hold the brake pedal and accelerate a little in gear. That will rock the engine. Try that in reverse too. If the smoke suddenly appears, suspect a worn rubber isolator in an engine mount. That will allow the engine to sit low and a pulley can contact a plastic shield.
Another possibility is a shorted radiator fan motor. I had a car at the local racetrack many years ago that had an electric fan. While minding my own business idling in the pits, all of a sudden I couldn't see through the windshield. A friend came over and offered his fire extinguisher thinking I was on fire. Turns out the smoke was coming from the fan motor. There was no smell typical of burning wiring so at first I just thought it was getting foggy outside. Your fan will rarely run on the highway, but once the engine is warm, it will turn on when you slow down or stop moving.
Since you think there's an exhaust leak, there might be hot exhaust gas blowing onto plastic parts. The amount of smoke you described seems kind of extreme to be caused by this, but it's not something to overlook.
Friday, April 16th, 2010 AT 11:06 PM