Often the forward jumping is less of a problem w a power snatch than a snatch because w the snatch you're more focused on elevating the bar (i.e. getting things moving vertically) and on the snatch you're more focused on getting under it, which can mean that you end up cutting the extension short or not continuing to drive w the legs all the way through the extension for fear of slowing down your transition at the top.
As Josh said, practice the first pull, don't just do only hang or block lifts. Halting snatch deadlifts and pulls or complexes with both can be really helpful. On halting DLs, halt w the bar at the hip - practice moving the bar back off the floor and keeping it as close to your thighs as possible until it comes into full contact at the hips. When you get comfortable with this, you can do complexes w halting DL + snatch.
You can also try power snatch + snatch - on the snatch, make sure you're pulling the same way and still trying to turn the bar over and secure it overhead as high as you can, but then continuing smoothly into the squat (note, this doesn't mean over-pulling, it just means don't dive down into a squat and put yourself farther down then where you need to be to meet the bar).