91 Accord sputtering only at highway speeds
 

Hi,
I have a 91 accord, automatic, 300,000km(Canadian =)
Just bought it a few months ago and noticed that whenever I drive on the highway (70KM/H+) it sputters and shakes. If I press the gas pedal while its shaking nothing happens for about 10 seconds then it lunges forward and everything fine for a while longer. This never happens in the city and the vehicle doesnt fully stall. The person I bought it from gave me the receipts for the fuel pump and the oxygen sensor, both just over a year old. He said the main relay acts up sometimes when trying to start the car but I dont think that would cause this problem while driving.
The code that I get is 43- Fuel System Malfuction
Any help is greatly appreciated!
Thanks!!!

I'm still suspect of the O2.....you are getting a code for it. Why not test it.

Are there any exhaust leaks on the car.....manifold cracked, manifold gasket leaking, ect ?