my z750 has about 5k on it.
recently it sounds like its been sputtering a little, and when i try to keep it at a certain higher rpm eg. 6k rpm, it would fluctuate up and down.
do you guys think it needs new spark plugs?
It might not be a bad idea to pull the plugs, check the gap, and see how they look. Also make sure the wires are on tight and the coils are clean.
While the tank is off make sure the intake and filter are OK.
It sounds like bad gas though. Was it stored over the winter without stabil or anything in it? Swap out for some new gas and put some injector cleaner in it. Ams-Oil makes some good cleaner and thats what I use most the time.