stephen does this surging happen while driving? or all the time even sitting still?
actually after i posted the above question i ran across this tidbit of info
check your charging from the alternator
cat had a surging problem on thier nissan K21&25 engines caused by the alternator overcharging and sending too much voltage to the electrical components in the fuel system.
It should not be charging anywhere outside this range from 13.8v to 14.9v. If its charging over this then check the chassis and engine block ground... make sure you have good ground points there. If all is ok there and you still have an overcharging condition then they recommend having the alternator repaired.