I've had a recent issue with my 1990 B2600 RWD manual transmission Mazda pickup truck. I will start it cold and drive at highway speeds for about 10 - 15 minutes when all of a sudden a whooshing sound suddenly comes from under the cab and I cannot accelerate when I push on the gas pedal. I can somewhat maintain the speed I am at and am able to go another 10 minutes or so to my destination, but I cannot get above 60 - 70 mph. I can stop and accelerate normally from stop lights. Also, when I arrive at my destination I notice a burnt smell. I pop the hood and see smoke coming from the outside of the air intake manifold cover. The engine does not get hot (the temperature gage is where it normally runs). No warning lights ever come on. I have noticed a bad vibration that emanates from inside the muffler (from putting my hand on it when the engine is first cranked), but nothing else out of the ordinary.
Coolant system seems to be okay, and spark plugs and wires were replaced about 10,000 miles ago.
Any ideas what could be causing this?