Car surging while moving

[jt98boost]

I have a surging problem with my 98 cobra that only happens while the car is above 20mph. The car will idle fine but if you are going down the road and put it in neutral it will surge up and down constant until you slow down to about 15mph or so and then it will fix itself. I have a prom80mm mass air located in the lower fender, bbk 65mmthrottle body vortech supercharger unichip dyno program. The car doesn't seem to do it as bad untill it warms up and then you can really tell it. Anyone got any ideas???