Ninja 250 sputtering at high speeds?

I just bought the ninja 250, 89', it runs great until i level off at 40 to 60 mph. when I level off at these speeds it sputters and is very jerkey. If i accelerate it stops but starts right back up wehn i level off. Ive dumped the tank and refilled but no help.

You need to pull the petcock from the tank, it may be plugged (mostly). And if there is an inline-fuel filter, check, clean it or replace it. This would be an aftermarket filter not a factory part.If not there may be junk in the jets, so do the Seafoam thing. (a gallon of gas and a can of sea foam. run it for 30 minutes a day for a week, to let it remove all that varnish)Then replace the plugs, which may be another cause of rough running. (check the wires to the plugs, a cable may be irregularly shorting out or too much resistance to the spark.)