1999 SeaDoo Bombadier jetski with engine troubles?

[Adam L]

The jetski starts and idles fine but when you try to take off it will peak out at bout 7000 rpm which is normal, but then it will immediately drop down to around 3000 rpm with the throttle still all the way down. Now if you lets it set and idle for a couple of seconds without pushing on the throttle you can gun it again and it will do the same thing again. I've heard it could have the wrong plugs and i've checked on those and they are not the problem. But any suggestions would help.