hesitates and "bucks" on highway

A clogged fuel filter or Cat convertor can cause this.

Could be a lean misfire, which is corrected by the extra fuel added during full throttle. Or ignition.

A SIMPLE AND QUICK WAY TO TEST FOR A BAD SPARK PLUG WIRE OR DISTRIBUTOR CAP. USE A SPRAY BOTTLE (WINDEX TYPE SPRAY BOTTLE) FILLED WITH WATER.SPRAY THE DISTRIBUTOR CAP AND THE SPARK PLUG WIRES WITH WATER THEN DRIVE THE CAR AND ACCELLERATE QUICK FROM A STOP. IF BUCKING OCCURES YOU NEED A TUNE UP.

Try taking off each spark plug wire one at a time and wiping them clean. Make sure they click onto the plugs when put back. If is works it's a no cost solution.

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You didn't say what vehicle you have. I'd also try cleaning the throttle plate as when spooge appears on the TB can affect performance at or near 0 throttle angle such as when you let off accelerator when you come up to speed etc.

Could also be ignition....wires/plugs.

MB

ONE OTHER THING, THE CAR MAKES A VERY LOUD RATTLING NOISE, FROM UNDERNEATH AROUND AREA OF CONVERTER, I'VE READ THAT BROKEN CERAMIC IN CONVERTER IS A VERY POSSIBLE CAUSE WHEN IDLING, BUT IT GOES AWAY WHEN DRIVING....

Check for loose heat shield or mounting hardware around the cat. Very common.